Rename confusing approval rule subheading

What does this MR do?

Attempts to address the customer concerns @phikai relayed in #333836 (closed) about the naming of this subheading: https://docs.gitlab.com/ee/user/project/merge_requests/approvals/rules.html#approvals-for-protected-branches

From looking at the page in the UI, I believe we meant to say "Approval rules for specific branches" - that would explain the "All" option in the UI.

Changing a subheading requires updating the crosslinks to it, as well.

Related issues

Closes #333836 (closed)

Author's checklist

• Follow the Documentation Guidelines and Style Guide.
• Ensure that the product tier badge is added to doc's h1.
• Request a review based on the documentation page's metadata and associated Technical Writer.

To avoid having this MR be added to code verification QA issues, don't add these labels: feature, frontend, backend, ~"bug", or database

Review checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
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    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
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    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
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• Ensure a release milestone is set.