feat!: bump checkstyle, gradle, java, mockito, spotless

[meotch]

Summary of Changes

Upgrades dependencies

• checkstyle 10 → 13
• java 17 → 21
• gradle 7.6.3 → 8.14.3
• mockito-inline 5.2.0 -> mockito-core 5.21.0
• spotless 7.0.2 → 8.2.0

Fixes #MC-9647

Public API Additions/Changes

None

Downstream Consumer Impact

Major dependency upgrades, including move from Java 17 to Java 21. If using locally, update your local gradle wrapper version via ./gradlew wrapper --gradle-version=8.14.3

How Has This Been Tested?

Tested locally through entire path stack, on both java and kotlin connectors

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