Cleanup obsolete RequireKotlin annotations

They were used to prevent older compiler versions from using specific API, but now these versions are even older than the first compiler version (namely 1.6) that can read this stdlib.

libraries/stdlib/unsigned/src/kotlin/annotations/Unsigned.kt

@@ -23,5 +23,4 @@ import kotlin.internal.RequireKotlinVersionKind
 @MustBeDocumented
 @Target(CLASS, ANNOTATION_CLASS, PROPERTY, FIELD, LOCAL_VARIABLE, VALUE_PARAMETER, CONSTRUCTOR, FUNCTION, PROPERTY_GETTER, PROPERTY_SETTER, TYPEALIAS)
 @Retention(AnnotationRetention.BINARY)
-@RequireKotlin("1.2.50", versionKind = RequireKotlinVersionKind.COMPILER_VERSION)
 public annotation class ExperimentalUnsignedTypes