Prevent unmodified designs from being versioned

What does this MR do?

Issue https://gitlab.com/gitlab-org/gitlab-ee/issues/24080.

Our SaveDesignsService would previously version and commit designs files that were not modified since their last version.

These would appear as updates in the version history for the design.

Besides the version history becoming a bit inaccurate and silly looking, this may create future problems for us. For example, because annotations use the existing DiffNote classes, we get errors when producing the discussion threads for discussions started on designs that have not changed in their "diff". There may be other issues lurking if we allow unmodified files to be committed as git wouldn't normally let this happen and designs (and versions) are backed by a git repository.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers