Site Health: Add OPcache saturation detection with recommended improvement notice

[Sukhendu2002]

Trac ticket: https://core.trac.wordpress.org/ticket/64854

Use of AI Tools

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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