Site Health: Add a check for tables using the legacy utf8mb3 charset

[irozum]

Adds a new Site Health check that scans the site's own database tables ($wpdb->tables( 'all', true )) for columns still using the legacy utf8/utf8mb3 character set, which is limited to 3-byte characters and cannot store emoji or other 4-byte characters, and has been deprecated by MySQL. The check reports good when no such columns are found, and recommended with a list of the affected table.column pairs otherwise. The scanned table list is filterable via the new site_status_utf8mb3_usage_tables filter.

Trac ticket: https://core.trac.wordpress.org/ticket/66109

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: Implementation, tests, and PR description. Reviewed by Igor Rozum.

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