feat!: bump to gradle 8 and java 21

[meotch]

Summary of Changes

• Removed unused MessageErrorThrowableSerializer
• Bump to Gradle 8.14.3 and Java 21

Fixes MC-12414

Public API Additions/Changes

None

Downstream Consumer Impact

Consumer will need to leverage Gradle 8.14.3 and Java 21

How Has This Been Tested?

Verified MessageError Throwable already gets serialized/deserialized properly with snapshot releases used in Wedge and Abagnale by stubbing RemoteUserService endpoint to throw a MessageError from Wedge and verified it got serialized/deserialized properly on Abagnale

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works

[stevecl5]

@meotch The jitpack.yml JDK version should be updated as well. I think we have that in all the GitHub libraries, even the ones published to Maven Central (they are published to Jitpack as well).