Add MySQL 9.7 to the test matrix

[desrosj]

MySQL 9.7.0 LTS was released on April 21, 2026.

The testing matrices should be updated to reflect this. Also, there are now no MySQL innovation releases actively supported until a new one is published in a few months.

Trac ticket: https://core.trac.wordpress.org/ticket/64894

Use of AI Tools

None.

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[github-actions]

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[desrosj]

Merged in r62407.