Revert "Bump Android SDK to 2.21.0"

[marco-saia-datadog]

What does this PR do?

This PR reverts commit f33bd07.

Motivation

The Android SDK bump causes the following issue, blocking our release:

> A failure occurred while executing com.android.build.gradle.internal.tasks.CheckAarMetadataWorkAction
   > An issue was found when checking AAR metadata:
     
       1.  Dependency 'androidx.metrics:metrics-performance:1.0.0-beta02' requires Android Gradle plugin 8.6.0 or higher.
     
           This build currently uses Android Gradle plugin 8.5.0.

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests
• Make sure you discussed the feature or bugfix with the maintaining team in an Issue
• Make sure each commit and the PR mention the Issue number (cf the CONTRIBUTING doc)
• If this PR is auto-generated, please make sure also to manually update the code related to the change

[sbarrio]

🙇