build: replace FindBugs with SpotBugs

[arnabnandy7]

Thank you for opening a Pull Request! Before submitting your PR, there are a few things you can do to make sure it goes smoothly:

• Make sure to open an issue as a bug/issue before writing your code! That way we can discuss the change, evaluate designs, and agree on the general idea
• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

Fixes #2154 ☕️

This replaces the unsupported FindBugs engine and Maven plugin with SpotBugs while retaining the historical google-http-client-findbugs artifact ID for downstream compatibility.

The custom beta-API detector was updated for the current BCEL API while preserving its public API compatibility. Build configuration, exclusion filters, sample configuration, detector fixtures, and public Javadocs were updated accordingly.

Validation included:

• Full Java 8 reactor verification with tests skipped
• Clirr compatibility checks with no reported API changes
• Checkstyle validation
• Dependency verification confirming the obsolete FindBugs engine is no longer present
• Detector fixture verification for BETA_CLASS_USAGE, BETA_FIELD_USAGE, and BETA_METHOD_USAGE