Use javax.annotation.processing.Generated for Java 11 native client (#3637)

Before Java 9, the `Generated` annotation was part of JEE and in the
`javax.annotation` package. Starting with Java 9, it is now part
of the standard SDK, under the new `javax.annotation.processing`
package.

This change creates a custom Generated annotation template for the
native clientlib, since it is guaranteed to be run in Java 11+.

Author: Benjamin Douglas

modules/openapi-generator/src/main/resources/Java/libraries/native/generatedAnnotation.mustache

@@ -0,0 +1 @@
+{{^hideGenerationTimestamp}}@javax.annotation.processing.Generated(value = "{{generatorClass}}", date = "{{generatedDate}}"){{/hideGenerationTimestamp}}