Spring Add Javadoc since for MediaTypeAssert.isNotEqualTo()

This PR add a Javadoc @since tag for the MediaTypeAssert.isNotEqualTo(String) method.

See gh-32756

Comment From: sbrannen

Please note that MediaTypeAssert will first be released with 6.2 which is documented at the class level.

Thus, there is no need to add @since 6.2 on methods introduced when the class is introduced.