[webview_flutter] Bump version for republishing

[stuartmorgan]

The published version of 2.0.6 is not compatible with the current
Flutter stable (2.2) and earlier due to an issue with how Dart 2.14
(on the current Flutter master) creates archives. See
dart-lang/pub#3001

This is a no-op change to allow republishing from stable.

Fixes flutter/flutter#82803

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 Flutter Style Guide and the C++, Objective-C, Java style guides. (Note that unlike the flutter/flutter repo, the flutter/plugins repo does use dart format. See plugin_tool format)
• I signed the CLA.
• The title of the PR starts with the name of the plugin 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.
• I updated CHANGELOG.md to add a description of the change.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test exempt.
• All existing and new tests are passing.

[mvanbeusekom]

LGTM

[stuartmorgan]

Landing on red; we're investigating the issue with iOS tests on stable, but it's unrelated to this PR, and this fix is urgent to get landed and published since 2.0.6 is broken for everyone on stable+Windows.