Document minimum Android API level and add workflow to check compatibility

[Marcono1234]

Purpose

Resolves #2429

Description

This is based on the discussion in #2429

• Documents minimum Android API level of 21 for the next Gson version (2.11.0 probably)
  For older versions it claims the minimum API level is 19 because Animal Sniffer did not find any violations there for the latest code from main. If it turns out that previous versions were actually incompatible with API level 19 we will have to adjust the README.
• Executes Animal Sniffer (as separate GitHub workflow) to verify compatibility

Checklist

• New code follows the Google Java Style Guide
• If necessary, new public API validates arguments, for example rejects null
• New public API has Javadoc
  • Javadoc uses @since $next-version$
    ($next-version$ is a special placeholder which is automatically replaced during release)
• If necessary, new unit tests have been added
  • Assertions in unit tests use Truth, see existing tests
  • No JUnit 3 features are used (such as extending class TestCase)
  • If this pull request fixes a bug, a new test was added for a situation which failed previously and is now fixed
• mvn clean verify javadoc:jar passes without errors

[eamonnmcmanus]

Thanks, this looks great!