UnderlineNav2: Add aria-hidden and sr-only class support for a descriptive "More" button label

[broccolinisoup]

Addressing an accessibility sign-off review issue that described below.

The label of the "More" overflow button is not descriptive, and could easily result in multiple buttons with the same name if the component is used multiple times on the same page. The heading text should be added to the accessible name of the button, e.g. "More Repository navigation items" or "More: Repository navigation", e.g. using an off-screen span inside the button and hiding the existing label from screen reader users with aria-hidden. Note that, as with the counter, the visible text of "More" must be included in the accessible name verbatim.

Merge checklist

• Added/updated tests
• Added/updated documentation
• Tested in Chrome
• Tested in Firefox
• Tested in Safari
• Tested in Edge

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[changeset-bot]

🦋 Changeset detected

Latest commit: eaeff03

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

This PR includes changesets to release 1 package

Name	Type
@primer/react	Patch

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[github-actions]

size-limit report 📦

Path	Size
dist/browser.esm.js	78.89 KB (0%)
dist/browser.umd.js	79.54 KB (0%)

[joshblack]

Left a comment above for a structure question, let me know what you think!