chore(tombstone-library): preserve version.rb untouched when tombstoning

[torreypayne]

Updates toys/tombstone-library to leave the existing lib/**/version.rb file completely untouched when tombstoning a library, rather than modifying or regenerating it from a template.

Details

In monorepos managed by Release Please (e.g. google-cloud-ruby), release-please is responsible for bumping version.rb and generating changelogs upon merging a breaking change (feat!:).

Previously, toys tombstone-library deleted all files and generated a new version.rb from version-template.erb.

This change:

1. Updates delete_files to preserve the existing lib/#{namespace_dir}/version.rb file as-is without touching it.
2. Removes version-template.erb and the generation step for version.rb.
3. Removes unused version resolution helpers and gem_version arguments.

This ensures zero unnecessary diffs to version.rb during tombstoning, allowing Release Please to manage the SemVer major bump upon merge.