chore(multiple): deprecate old docker actions and add migration guide

[ricky-undeadcoders]

This pull request introduces deprecation notices for the build-push-to-dockerhub and push-to-gar-docker GitHub Actions, guiding users to migrate to the new docker-build-push-image action. It updates documentation and workflow files to provide clear migration instructions and runtime warnings.

Deprecation and migration guidance:

• Added deprecation warnings to the top of the README.md files for both actions/build-push-to-dockerhub and actions/push-to-gar-docker, informing users of the removal timeline and the recommended migration path. [1] [2]
• Inserted a new "Migrating" section in actions/docker-build-push-image/README.md with step-by-step instructions and code examples for migrating from both deprecated actions to docker-build-push-image.

Workflow runtime warnings:

• Added a deprecation warning step in both actions/build-push-to-dockerhub/action.yaml and actions/push-to-gar-docker/action.yaml to display a warning message during workflow execution, ensuring users are notified even if they miss the documentation updates. [1] [2]