Replacing project references with container name in repository classes

What does this MR do?

In !21739 (merged), !23164 (merged), !23494 (merged) we're starting the process of adding repositories to snippets. In this MR, we start replacing the term project (bound to git related classes like Commit, Blob, ...) with container since now the object of the git object can be also a snippet.

In this MR we refactor the related classes to use this term and we extract some repository related methods to a concern.

Refs #39176 (closed)

Does this MR meet the acceptance criteria?

Conformity

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