ci: update actions/checkout to v7

[katsugtgz]

Updates the CI workflow to use current action versions.

Changes:

• actions/checkout@v2 -> actions/checkout@v7 in ci.yml

Validation:

• workflow YAML parses after the change
• setup-python and other actions in the file are left unchanged

Scope: .github/workflows/ci.yml only.

Note: the first push of this branch set v4; the follow-up force-push brought it to the current major, v7. Checks need maintainer approval to run on this fork PR.

[jugglerchris]

Hi - sorry for the delay (summer holidays).
What's the reason for updating the action?

[katsugtgz]

Hi. I updated actions/checkout from v2 to v7 for these reasons:

1. checkout v2 uses a Node runtime that GitHub no longer supports.
2. GitHub documents the current major as the supported version.
3. The change keeps CI on a maintained action and reduces the risk of future workflow failures.
   The workflow behavior stays the same. Only the action major version changes.

[jugglerchris]

Thanks for the contribution, but I'm going to close this MR. No doubt it's well intentioned, but I don't see a strong motivation to change what works at this point - and to be honest I still don't understand why this PR for something which shouldn't affect anyone other than me!