Bump Android SDK to 2.21.0 and fix android build errors on RN <0.76

[sbarrio]

What does this PR do?

• Bumps the native Android SDK dependency to 2.21.0.
• Fixes a build issue related to the inclusion of androidx.metrics:metrics-performance:1.0.0-beta02:

 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.

The solution for this has been to force the resolution to androidx.metrics:metrics-performance:1.0.0-beta01 when building for React Native versions under 0.76.0.

This dependency was original bumped here: DataDog/dd-sdk-android#2546

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

[datadog-datadog-prod-us1]

Datadog Report

Branch report: sbarrio/dev/bump-android-sdk-to-2.21.0-fix-pre-rn-76-build-error
Commit report: 67c2a8c
Test service: dd-sdk-reactnative

✅ 0 Failed, 663 Passed, 1 Skipped, 3.74s Total Time

[sbarrio]

@0xnm Pinging you here to double check if this approach seems reasonable enough or if you think we should explore an alternative to get Android builds on RN 0.76 to work with newer versions of dd-sdk-android 🙇

[marco-saia-datadog]

LGTM 👍