BREAKING CHANGE: Experimental features are no longer built into release images.
Users must now build custom images with specific features if needed.

- Remove experimental build job from release workflow
- Remove experimental features from CI matrix
- Update documentation to reflect new feature strategy
- Simplify Dockerfile by removing experimental feature args
- Keep RUN_ALL_EXPERIMENTAL_TESTS for testing purposes only

Migration: Users who need specific features should:
1. Build custom images using --build-arg FEATURES="feature1 feature2"
2. Use the documented feature flags in their builds
3. Refer to the new documentation for building with custom features

Closes  #9210
Closes #9204
Partially #7415