Add Javadoc since for JavaVersion.TWENTY_ONE

[izeye]

This PR adds a Javadoc @since tag for the JavaVersion.TWENTY_ONE.

See gh-35892

[wilkinsona]

Nearer the release of Java 21, we'll add TWENTY_ONE to the enum in 2.7.x. With the final RC of Java 21 on 24 August, the same day as 2.7.15 is released, that'll probably happen in 2.7.16. We can add @since at that point. This will be before 3.2 GAs so I don't think there's much point in applying this change now. Thanks anyway.

I've also opened #36608 as many of the existing values are missing @since in their javadoc.