Updates Gradle version for Flutter project templates and integration tests to at least 7.6.3 (changed all of those with versions below it) to fix security vulnerability.
Part of fix for https://github.com/flutter/flutter/issues/138336.
* Revert "Mark last failing test after gradle update as flaky. (#91423)"
This reverts commit 46a52d03bd.
* Revert "fix android template for Gradle 7 (#91411)"
This reverts commit 51d06d537f.
* Revert "Add explicit version for mac and windows openjdk. (#91408)"
This reverts commit bf429f2771.
* Revert "Update the openjdk version used by linux android tests. (#91405)"
This reverts commit 2144ab8b45.
* Revert "Migrate to Gradle 7.0.2 / AGP 7.0.1 (#90642)"
This reverts commit b6459f9b63.
