Improve GitHub Actions

[HylianFreddy]

• Update dependencies to latest version to run on Node.js 24, fixing warnings about deprecation of Node.js 20 actions
  • actions/checkout@v6.0.2,
  • metcalfc/changelog-generator@v4.7.0,
  • ncipollo/release-action@v1.21.0 (now requires explicit contents: write permission)
• Drop dependencies on exuanbo/actions-deploy-gist and EndBug/add-and-commit as those operations are easy to implement with simple shell commands, and the first one doesn't run on Node.js 24 on its latest version.
• Make the deploy-gist job run only on the main repo, so that making a Release from a fork won't show this step as failed.
• Change the message for the formatting commit to be shorter ("Auto-format code to match clang rules"); I think this is better considering it should essentially never appear on the main branch with the PR checks currently in place.
• Add a step to the Confirm Compilable job to upload the artifact built during the PR check, so it can be downloaded from the Checks tab on the PR page.