fix(theme-classic): consistently apply the right active class name for all navbar items

[Josh-Cena]

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

Inspired by #7404, I realized we are not apply the right class name for the locale dropdown. This PR fixes it by avoiding code duplication.

Test Plan

Test links

Deploy preview:

The locale link is now properly highlighted

Related issues/PRs

[netlify]

✅ [V2]

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🔨 Latest commit	273f35b
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[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

URL	Performance	Accessibility	Best Practices	SEO	PWA	Report
/	🟠 84	🟢 100	🟢 100	🟢 100	🟢 90	Report
/docs/installation	🟠 87	🟢 99	🟢 100	🟢 100	🟢 90	Report

[github-actions]

Size Change: -192 B (0%)

Total Size: 796 kB

Filename	Size	Change
website/build/assets/js/main.********.js	598 kB	-192 B (0%)

ℹ️ View Unchanged

Filename	Size
website/.docusaurus/globalData.json	52.3 kB
website/build/assets/css/styles.********.css	106 kB
website/build/index.html	38.9 kB

_{compressed-size-action}

[slorber]

👍 looks like it works fine and happy to remove this utils file even if it duplicates a bit