Clarify when breaking changes happen in API versions

What does this MR do?

This MR makes our REST API docs less ambiguous about when breaking changes can happen.

Previously, it was ambiguous about whether breaking changes could happen only on a major API version change, or a major GitLab version change.

On one hand:

The HTTP API is versioned with a single number, which is currently 4. This number symbolizes the major version number, as described by SemVer. Because of this, backward-incompatible changes require this version number to change.

But on the other hand:

Backward-incompatible changes (for example, endpoint and parameter removal), and removal of entire API versions are done in tandem with major GitLab releases.

This change makes the second paragraph less prone to misinterpretation.

Related issues

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