[Fix] Set minSdkVersion on core gradle.properties to 21

[sbarrio]

What does this PR do?

This PR reverts the default minSdkVersion set on core's gradle.properties to 21

Motivation

We should keep the minSdkVersion at 21, not 33 (sorry).

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/fix/set-default-minsdkversion-to-21
Commit report: 06146eb
Test service: dd-sdk-reactnative

✅ 0 Failed, 669 Passed, 1 Skipped, 3.79s Total Time