bump github checkout action to v4

[hkershaw-brown]

Description:

GitHub is deprecation Node.js 16 actions, this pull request updates the checkout action to v4

Fixes issue

fixes #630

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Documentation update
• [x] Github actions update

Documentation changes needed?

• [ ] My change requires a change to the documentation.
  • [ ] I have updated the documentation accordingly.

Tests

Please describe any tests you ran to verify your changes. Testing with a pull request to the rttov-clouds branch.

Checklist for merging

• [ ] Updated changelog entry
• [ ] Documentation updated
• [ ] Update conf.py

Checklist for release

• [ ] Merge into main
• [ ] Create release from the main branch with appropriate tag
• [ ] Delete feature-branch

Testing Datasets

• [ ] Dataset needed for testing available upon request
• [ ] Dataset download instructions included
• [ ] No dataset needed