Use the legacy `Gemfile`/`Gemfile.lock` names in `tooling/overcommit`

What does this MR do?

This is because when multiple versions of Bundler are installed (e.g. 1.17.3 and 2.1.4), Bundler will choose the 2.x version based on the gems.rb / gems.locked file names but it chooses the 1.17.x version if the we use the legacy Gemfile / Gemfile.lock file names.

For now, we'll use the legacy file names to be consistent with the main gitlab's Gemfile.lock and to avoid unexpected changes in the working tree.

After this is merged, people will need to run make -C tooling/overcommit to update their .overcommit.yml file.

Does this MR meet the acceptance criteria?

Conformity

• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Separation of EE specific content