Update checkout action to v7

[luojiyin1987]

Fixes #34065.

Summary

All workflows now use actions/checkout@v7.
This replaces two v4 references and one v3 reference.
The v7 tag tracks compatible version 7 releases.

Impact

The repository now uses one checkout major version.
Future compatible version 7 releases need no manual update.
No checkout inputs changed.
No workflow permissions changed.
No workflow commands changed.

Checks

• Parsed all three changed workflows as YAML.
• Confirmed that the official v7 tag exists.
• Ran git diff --check.
• Confirmed that the diff changes only three version references.

[krystian3w]

But won’t that mean that when a compatible version of 7.0.2/7.1/7.2 comes out, we’ll have to bump it again?

[luojiyin1987]

Good point. I’ve changed it to actions/checkout@v7, so compatible v7 updates will be picked up automatically.