Editor: restore the behavior of only showing back button focus ring on :focus-visible

[fushar]

What?

In #48472 we only show the focus ring for editor sidebar back button on :focus-visible only. Then #80029 removes that behavior by mistake (sorry!). This PR reintroduces the behavior, but not by reverting as-is and instead proposing a simpler CSS. So in the end we still get a cleaner CSS in the file 😄

Testing Instructions

1. Go to Site Editor.
2. Click Pages (via mouse).
3. Verify the chevron back button (< Pages) doesn't get focused (you don't see focus ring).

Use of AI Tools

Used Opus 4.8 to brainstorm solutions.

[github-actions]

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Co-authored-by: fushar <fushar@git.wordpress.org>
Co-authored-by: t-hamano <wildworks@git.wordpress.org>

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[github-actions]

Size Change: +39 B (0%)

Total Size: 7.69 MB

📦 View Changed

Filename	Size	Change
build/styles/edit-site/style-rtl.css	21.3 kB	+10 B (+0.05%)
build/styles/edit-site/style-rtl.min.css	17.5 kB	+9 B (+0.05%)
build/styles/edit-site/style.css	21.3 kB	+11 B (+0.05%)
build/styles/edit-site/style.min.css	17.5 kB	+9 B (+0.05%)

_{compressed-size-action}

[github-actions]

Flaky tests detected in 6b18e98.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/29100080783
📝 Reported issues:

• [Flaky Test] adds and activates a new tab when pressing Enter at the end of a tab label #80106 in /test/e2e/specs/editor/blocks/tabs.spec.js

[t-hamano]

Thanks for the PR!

The current behavior is not strictly a bug, as it is based on a design system decision to prioritize the use of :focus over :focus-visible. However, I understand the feedback that it is bothersome to see the focus ring appear every time you move across the screen.

For now, I think we can ship this PR to restore the previous behavior.