BLD: Upgrade GitHub actions that use deprecated node12

[rrjbca]

Description

The GitHub actions checkout@v2, setup-python@v2 and codecov-action@v1 all raise the warning "uses node12 which is deprecated". This PR upgrades the skypy workflows to use checkout@v4, setup-python@v4 and codecov-action@v3 which uses the compliant node16.

Checklist

• [x] Follow the Contributor Guidelines
• [ ] Write unit tests
• [ ] Write documentation strings
• [ ] Assign someone from your working team to review this pull request
• [x] Assign someone from the infrastructure team to review this pull request