Bump legacy all_packages project AGP version to 7.0.0, Gradle version to 7.0.2

[camsim99]

Upgrades legacy all_packages project AGP version to 7.0.0 and Gradle version to 7.0.2 as Flutter will begin enforcing supported versions for these Android dependencies.

This was prompted by dependabot AGP version upgrades like #6522 that are failing due to a what a clear warning* describes as a potential issue:

Warning: Flutter support for your project's Gradle version (6.7.1) will soon be dropped. Please upgrade your Gradle version to a version of at least 7.0.2 soon.
Alternatively, use the flag "--android-skip-build-dependency-validation" to bypass this check.

Potential fix: Your project's gradle version is typically defined in the gradle wrapper file. By default, this can be found at /b/s/w/ir/x/w/packages/legacy/all_packages/android/gradle/wrapper/gradle-wrapper.properties. 
For more information, see https://docs.gradle.org/current/userguide/gradle_wrapper.html.

Warning: Flutter support for your project's Android Gradle Plugin version (4.1.0) will soon be dropped. Please upgrade your Android Gradle Plugin version to a version of at least 7.0.0 soon.
Alternatively, use the flag "--android-skip-build-dependency-validation" to bypass this check.

Potential fix: Your project's AGP version is typically defined the plugins block of the `settings.gradle` file (/b/s/w/ir/x/w/packages/legacy/all_packages/android/settings.gradle), by a plugin with the id of com.android.application. 
If you don't see a plugins block, your project was likely created with an older template version. In this case it is most likely defined in the top-level build.gradle file (/b/s/w/ir/x/w/packages/legacy/all_packages/android/build.gradle) by the following line in the dependencies block of the buildscript: "classpath 'com.android.tools.build:gradle:<version>'".

Other dependabot upgrades that I believe are blocked by this change:
#6585
#6534
#6530
#6528
#6526

*Added in flutter/flutter#143341

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/packages repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[reidbaker]

I think this work is also related to flutter/flutter#146660 because AGP 8.3 being used in a package prevents adoption in the legacy project.

[camsim99]

I believe this also addresses what flutter/flutter#137181 is about and should address concerns about modifying the legacy all_packages app noted in #5032 (comment).

[stuartmorgan]

This was prompted by dependabot AGP version upgrades like #6522 that are failing due to a what a clear warning* describes as a potential issue:
[...]
*Added in flutter/flutter#143341

That's not on stable yet. Do we really want the legacy test no longer catching problems that will break people on stable?

I would have expected this PR to wait until the next stable release.

[stuartmorgan]

Also, I would recommend that we set the minimum Flutter SDK version for all *_android packages to the version with that error message before we lose this test coverage. Otherwise when we start landing dependabot updates that break people (intending to rely on the new warning), people on 3.19 and 3.16 will also be broken, but without the useful messages.

[reidbaker]

If @gmackall's work to warn on old versions is not in stable I think we need to revert this pr.

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this issue is being reverted.

[camsim99]

Reason for revert: Warning about unsupported Android dependency versions (added in flutter/flutter#143341) not in stable.

[stuartmorgan]

I've done all the repo updates for 3.22, so it should be possible to make an updated version of this PR that makes affected plugins require 3.22 now.

[camsim99]

@stuartmorgan that would be changing the Flutter version for all packages that run on Android right (excluding platform interfaces I assume)?

[stuartmorgan]

You only need to change the min SDK actual Android implementation packages. It's fine for people to use newer versions of the app-facing packages with older versions of the implementation package (up until the point where someone updates the min version of the Android implementation package in the app-facing package, at which point CI will force them to update the app-facing package's minimum accordingly).