Build/Test Tools: Trim the routine PHPUnit database matrix in the 6.8 branch

[lancewillett]

Trac ticket: https://core.trac.wordpress.org/ticket/66069

Carries the routine MariaDB trim from r63584 to the 6.8 branch. The MySQL list already matches trunk's routine set.

• MariaDB: 5.5, 10.6, 10.11, and 11.4 on pushes and pull requests. 10.3, 10.4, and 10.5 run on dispatched runs. 11.8 is not in this branch's list, so it is not added.

Scheduled runs only fire from the default branch, so this keys the full list on workflow_dispatch rather than schedule. test-old-branches.yml dispatches this branch's PHPUnit workflow monthly, so the dropped versions keep running there.

MySQL and MariaDB jobs on a pull request: 87 → 63. Dispatched runs: 87, unchanged.

Validation

• Expanded both matrices for pull_request and workflow_dispatch, before and after: 87 → 63 and 87 → 87.
• actionlint and zizmor --persona=regular report the same findings before and after.

Use of AI tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: Measuring job counts, the workflow edit, matrix validation, and drafting this description. Reviewed by me.

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This pull request is for code review only. Please keep other discussion in the Trac ticket. Do not merge this pull request.

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[desrosj]

Same comment as on #13540

The changes themselves look fine. However, I find the new syntax to be pretty hard to read. I've opened #13547 to propose a different syntax to accomplish the same thing. Not a hill I am going to die on, but happy for you to at least consider this as a suggestion to improve readability.

And as well as the other PRs:

Is it intentional to not support the full matrix label in this branch?

[lancewillett]

#13547