Reverts "Bump legacy all_packages project AGP version to 7.0.0, Gradle version to 7.0.2 (#6591)"

[auto-submit]

Reverts: #6591

Initiated by: camsim99

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

Original PR Author: camsim99

Reviewed By: {reidbaker, gmackall}

This change reverts the following previous change:
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.