Do not convert return types in NoGuavaJava21 by default

[BramliAK]

What's changed?

Rollback visit Variable Declarations

Checklist

• I've added unit tests to cover both positive and negative cases
• I've read and applied the recipe conventions and best practices
• I've used the IntelliJ IDEA auto-formatter on affected files

[github-actions]

Some suggestions could not be made:

• src/main/java/org/openrewrite/java/migrate/guava/AbstractNoGuavaImmutableOf.java
  • lines 19-19

[timtebeek]

hi @BramliAK ; Understand that you'd like this fixed; Instead of completely removing this, can we make this opt-in with an @Option instead? That way the nice addition you added in #590 isn't completely lost, and it opens the door to an alternative composite recipe that does potentially unsafe replacements.

[timtebeek]

Thanks @BramliAK ! Good to be back to stable recipe outputs by default, while still being able to do the unsafe migrations if desired.