Update build.gradle with a comment explaining version (flutter#3972)

Followup to
https://github.com/flutter/packages/pull/3960/files/eda00fe0ee329433a013b63a1b7727f90f2d0de2#r1192563559

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] 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`.)
- [x] 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 listed 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 `///`).
- [x] I added new tests to check the change I am making, or this PR is
[test-exempt].
- [x] All existing and new tests are passing.

packages/camera/camera_android_camerax/example/android/build.gradle

@@ -1,4 +1,7 @@
 buildscript {
+    // This version should intentionally be a 1.7.* version and lower than the 
+    // version of kotlin-bom defined in packages/camera/camera_android_camerax/android/build.gradle.
+    // This tests that the kotlin version resolution continues to work. 
     ext.kotlin_version = '1.7.10'
     repositories {
         google()