This PR upgrades Gradle to 7.4 & AGP to 7.1.1.

It includes the following:

• Gradle version upgraded to 7.3.3 with the ./gradlew wrapper --gradle-version=7.3.3 --distribution-type=all command.
• AGP version upgrade to 7.0.4 (see settings.gradle change).
• Build output diff:
  • To identify and fix new warnings/errors that got added.
  • To identify and verify old warnings/errors that got removed.
• Lint output diff
  • To identify and fix new warnings/errors that got added.
  • To identify and verify old warnings/errors that got removed.
• The above 4 steps were redone with a follow-up update to Gradle 7.4 & AGP 7.1.1.
• Update min sdk version to 21.
• Update buildkite docker android build image to 1.2.0.

It suppresses the following Lint issues:

• Unused resources for dimens.

It resolves the following Lint issues:

• Obsolete sdk int version checks.

Besides the above, you can reference the Gradle 7.4 & AGP 7.1.1 PRs of the below clients for testing/verification purposes:

• WordPress-Android

NOTE: This is a draft PR because all Gradle & AGP upgrade PRs need to be merged together.

Merge Instructions

• Mark PR as Ready for review and assign https://github.com/orgs/wordpress-mobile/teams/owl-team to it.
• Wait for PR to get approved.
• Merge this PR (in coordination with all the other library PRs).

To Test

1. CI checks will cover everything in terms of testing.
2. Testing composite builds and respective clients will be enough (see above).