Consider supporting assignment overload for ConfigurableFileCollection

[lptr]

Let's check out if it works already with Groovy. Even if we don't support FileCollection = [File], we could consider adding it.

Let's consider supporting += in Kotlin as well. Groovy already supports FileCollection += FileCollection.

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cc: @gradle/bt-execution

[asodja]

Note that Groovy doesn't support ConfigurableFileCollection = FileCollection or ConfigurableFileCollection += FileCollection. It throws:

Caused by: groovy.lang.GroovyRuntimeException: Cannot set the value of read-only property 'col' for task ':app2:myTask' of type MyTask.
        at org.gradle.internal.metaobject.AbstractDynamicObject.setReadOnlyProperty(AbstractDynamicObject.java:134)
        at org.gradle.internal.metaobject.BeanDynamicObject$MetaClassAdapter.setProperty(BeanDynamicObject.java:372)
        at org.gradle.internal.metaobject.BeanDynamicObject.trySetProperty(BeanDynamicObject.java:181)
        at org.gradle.internal.metaobject.CompositeDynamicObject.trySetProperty(CompositeDynamicObject.java:66)
        at org.gradle.internal.metaobject.AbstractDynamicObject.setProperty(AbstractDynamicObject.java:74)
        at MyTask_Decorated.setProperty(Unknown Source)

[asodja]

Note: for += there is a different issue now: #23637. This will be tackled later.