fix(theme-classic): add caret for dropdown on mobile

[lex111]

Motivation

After merging of #6983, the caret icon on collapsible dropdown is no longer displayed on mobiles

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Yes

Test Plan

Related PRs

[netlify]

✅ [V2]

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	58
🟢 Accessibility	100
🟢 Best practices	92
🟢 SEO	100
🟢 PWA	90

Lighthouse ran on https://deploy-preview-7048--docusaurus-2.netlify.app/

[github-actions]

Size Change: +26 B (0%)

Total Size: 807 kB

ℹ️ View Unchanged

Filename	Size	Change
website/.docusaurus/globalData.json	49.9 kB	0 B
website/build/assets/css/styles.********.css	105 kB	0 B
website/build/assets/js/main.********.js	613 kB	+26 B (0%)
website/build/index.html	38.4 kB	0 B

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