Move snippet feature flag check from git access class to controllers

What does this MR do?

At the moment, class GitAccessSnippet ensures that the feature flag :version_snippets is enabled in order to allow/disallow snippet git actions.

Nevertheless, there are git operations, like when we update the snippet info, that we want to perform always, no matter the feature flag status.

We need to move the check outside the access class into the controllers and the internal API.

Refs #205666 (closed)

Does this MR meet the acceptance criteria?

Conformity

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