Downgrade actions/checkout to v5

[mattsu2020]

What changed

Downgraded every GitHub Actions workflow use of actions/checkout from v6 to v5.

Why

actions/checkout@v6 can fail during checkout before any project code runs with this error:

fatal: could not read Username for https://github.com: terminal prompts disabled

This follows the workaround discussed in #12446 while preserving the existing persist-credentials: false setting.

Impact

Workflow checkout behavior returns to the prior stable action version while keeping credentials from being persisted in the local git config.

Validation

• Confirmed no actions/checkout@v6 references remain under .github
• Confirmed all checkout references under .github now use actions/checkout@v5

Fixes #12446

[codspeed-hq]

Merging this PR will not alter performance

✅ 319 untouched benchmarks
⏩ 46 skipped benchmarks¹

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_{Comparing mattsu2020:fix_12446 (9687cfa) with main (4f9a39b)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[mattsu2020]

This is off-topic, so I'm closing it.