Fix kotlin source/target version.

[sdedic]

The 4.0 upcoming version of Micronaut (now M2) uses an upgraded version of Kotlin gradle plugin that somewhat breaks its backward compatibility.
According to KT-32805, Kotlin compiler task used to inherit from SourceTask that has sourceCompatibility and targetCompatibility properties, but from version 1.7 it derives just from DefaultTask.

According to that report, the plugin ignored the *Compatibility properties even before that. The proper settings reside in kotlinOptions structure.

This resulted in

Reason: groovy.lang.MissingPropertyException: Could not get unknown property 'sourceCompatibility' for task ':aop:compileKotlin' of type org.jetbrains.kotlin.gradle.tasks.KotlinCompile.
groovy.lang.MissingPropertyException: Could not get unknown property 'sourceCompatibility' for task ':aop:compileKotlin' of type org.jetbrains.kotlin.gradle.tasks.KotlinCompile.
	at org.gradle.internal.metaobject.AbstractDynamicObject.getMissingProperty(AbstractDynamicObject.java:85)
	at org.gradle.internal.metaobject.AbstractDynamicObject.getProperty(AbstractDynamicObject.java:62)
	at org.gradle.api.internal.AbstractTask.property(AbstractTask.java:571)
	at org.gradle.api.DefaultTask.property(DefaultTask.java:169)
	at org.netbeans.modules.gradle.tooling.NbProjectInfoBuilder.detectSources(NbProjectInfoBuilder.java:1088)

This PR makes the kotlinOptions the primary source for source/target version kotlinCompile task; falls back to the previous behaviour. In addition task properties are fetched by a safe helper that does not throw on a missing property. The procedure should be safe for all versions of Kotlin plugin: should they not expose kotlinOptions.{jvmTarget|languageLevel}, it will be (safely) taken from {source,target}Compatibility properties of the task.

Fixes #5896

[lkishalmi]

Looks good to me!
Thank you for fixing this one!