Fix java 21

[wetted]

Supersedes #771 to stop micronaut-build force pushing it all the time

[wetted]

This build does not like the toolchain configuration. I've used this on numerous modules already. Not sure why it's a problem here:

kotlin {
    jvmToolchain {
        languageVersion.set(JavaLanguageVersion.of(17))
    }
}

   > A problem occurred configuring project ':test-suite-kotlin'.
      > A problem occurred evaluating project ':test-suite-kotlin'.
         > The value for property 'languageVersion' is final and cannot be changed any further.

Maybe another plugin is setting it internally? 🤷‍♂️

[timyates]

This is only for tests so I think it's ok... But we then need some things for graal

[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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[timyates]

If this is for the 4.2.0 release, should we bump the minor version, and use the latest 4.1.x core and platform dependencies?