Add Kotlin jvmTarget

Thore Göbelrequested to merge
thgoebel/fdroidclient:jvmTarget into master

Without this, the build fails for me with (likely because my installation defaults to Java 21):

Execution failed for task ':app:compileFullDebugKotlin'.
> Inconsistent JVM-target compatibility detected for tasks 'compileFullDebugJavaWithJavac' (17) and 'compileFullDebugKotlin' (21).

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