[ci] Bump actions/setup-python to v5.1.1

[timhirsh]

Summary

See #260 for background.

Update the actions/setup-python action to the most recent version: v5.1.1. Quick summary of major versions:

• v4: there is no longer a default python-version set in the Action, so implementers must specify one. We already specify one, so we should be ok.
• v5 contained the Node20 upgrade

Test Plan

• Verify new hash on this PR
• Checked job summary and saw no deprecation warnings related to actions/setup-python
• Checked the changelog and didn't see any backwards incompatible changes

[chiphogg]

Double checked hash from release.

It's pretty rare that Au feels the need for a submit queue, but this would have been one of those times. 🙂