Do not populate dynamic label with active item in multi-select mode

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What are you trying to accomplish?

This PR fixes an issue causing the ActionMenu's dynamic label from being set to one of the selected items on page load. For single-select mode, this behavior is desirable, but the dynamic label feature is not supported in multi-select mode and should not appear on page load.

Screenshots

See linked issue.

Integration

No changes necessary in production.

List the issues that this change affects.

Fixes https://github.com/github/primer/issues/3082

Risk Assessment

• Low risk the change is small, highly observable, and easily rolled back.
• Medium risk changes that are isolated, reduced in scope or could impact few users. The change will not impact library availability.
• High risk changes are those that could impact customers and SLOs, low or no test coverage, low observability, or slow to rollback.

Accessibility

• No new axe scan violation - This change does not introduce any new axe scan violations.

Merge checklist

• Added/updated tests
  - [ ] Added/updated documentation
• Added/updated previews (Lookbook)
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

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🦋 Changeset detected

Latest commit: a1565fc

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/view-components	Patch

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