Bump JDK running Gradle from 11 to 17

[rognan]

This commit decouples the JDK running Gradle from the one building the
project.

Gradle will by default use the same JDK for building the project as it
does for running Gradle. Newer JDKs are assumed to be more performant,
contain fewer bugs and to come with desirable features that we would
like to utilize in our build, but doing so should not endanger
backwards compatibility for the plugin itself. We can do that now by
utilizing Gradle toolchains, which allows us to use one JDK/JRE for
running Gradle and the other for building the project, i.e. executing
`javac`.

Doing this requires us to make multiple JDKs available to the Gradle
build action for each individual execution of the build. The setup-java
action doesn't support this at the moment, and the multiple build steps
configured for actions/setup-java in this commit is at best a
workaround for this, and at worst a bad assumption with respect to
environment variables or other settings being overwritten. Furthermore,
there's no handling of duplicate invocations of the action.

Hopefully both actions/setup-java and gradle/gradle-build-action will
handle all of this gracefully.