Update GitHub Actions CI

[striezel]

This PR updates outdated actions in the GitHub Actions workflows.

The following updates are performed:

• update actions/cache to v4
• update actions/checkout to v4
• update actions/upload-artifact to v4

Still using the outdated actions will generate several warnings in CI runs, for example in https://github.com/WinMerge/winmerge/actions/runs/8879019459:

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/checkout@v3, actions/cache@v3, actions/upload-artifact@v3. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

The PR will get rid of those warnings, because the newer versions of those actions use Node.js 20.

[sdottaka]

Thank you!