Update GitHub Actions workflow

[klu2]

Gradle:

• update to latest Gradle version (7.4), there was 4.10 before
• convert to Kotlin DSL for the build-scripts, therefore being typesafe
• add Class-Path to the JAR manifest
• making signing locally optional so that you can safely run gradle build locally

I'd also recommend you to use in-memory ascii-armored keys on the CI server as explained https://docs.gradle.org/current/userguide/signing_plugin.html#sec:in-memory-keys. You can have a look at https://github.com/cloudflightio/json-wrapper/blob/master/build.gradle.kts#L97 how I did that in another repository, just pass the PGP_SECRET and PGP_PASSPHRASE as environment variables then.

Gitlab Actions:

• build on all pushes in order to have build checks also for pull requests
• use temurin instead of adopt, see https://blog.adoptopenjdk.net/2021/08/goodbye-adoptopenjdk-hello-adoptium/
• use gradle images to validate wrapper and build, see https://help.github.com/actions/language-and-framework-guides/building-and-testing-java-with-gradle

I'd also recommend you to create a separate GitHub Action Pipeline to push to maven central directly by creating a Release here on GitHub, see https://github.com/cloudflightio/json-wrapper/blob/master/.github/workflows/github-publish.yml