ci: update GitHub Actions versions

[vincenzopalazzo]

Summary

• update all workflows to use the latest major of actions/checkout
• update actions/cache and peter-evans/create-pull-request where newer majors are available
• set explicit read-only contents permissions on non-mutating workflows

Testing

• go run github.com/rhysd/actionlint/cmd/actionlint@latest
• git diff --check

Closes #155

[ldk-reviews-bot]

I've assigned @benthecarman as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[benthecarman]

Can we also add so we don't get duplicate runs for multiple pushes

concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}
  cancel-in-progress: true