Change minimum supported Java version to 21

[cicirello]

Summary

Sometime after Java 21 (the next LTS version of Java) is released, increase minimum supported Java version to 21. This includes the following:

• Changing the Java version within the pom.xml
• Changing the Java version in all GitHub Actions workflows
• Updating the README and other documentation
• Any updated necessary to do so based on any relevant changes to the Java API

Note that this will be a breaking change.