Migrate to Node 20 on CI via GitHub Actions major upgrades

[TWiStErRob]

Reduce warnings:

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: actions/checkout@v3, gradle/wrapper-validation-action@v1, actions/setup-java@v3, actions/cache@v3, gradle/gradle-build-action@v2. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

wrapper-validation doesn't have a version yet: gradle/wrapper-validation-action#160 (it's quite possible it'll end up as gradle/actions/wrapper-validation@v2 in a few weeks)

Note: most actions treat Node 20 as major bumps because self-hosted runners break down from automatic minor bumps via @vX, if there's no Node 20 installed on them.

[TWiStErRob]

Please run the manual workflow against this branch before merging.

[ychescale9]

Thanks.

Please run the manual workflow against this branch before merging.

It doesn't look like I'm able to target your branch from the manual workflow UI 🤔

[TWiStErRob]

Oops, you're right, the only thing you can do is add my fork as remote and push a branch to your origin. Or merge blindly and 🤞 😉.

[ychescale9]

Cool, I'll merge then as the main workflow should cover most if not all your changes.