[21u-communication] JDK 21u process changes

[Severin Gehwolf]

Hi,

Please pause submitting backport PRs for JDK 21u until further notice.
I intend to set up a similar process for JDK 21u as we have for JDK
17u. That is, we'll be splitting jdk21u into jdk21u-dev and jdk21u. The
jdk21u-dev repo needs to get created and that's where we'll be
collecting 21.0.3 (April 2024) changes. jdk21u will be the ramp-down
repo for the April 21.0.3 release when the time comes. That should be
no different than we do for other older JDK releases.

I'll update this list once this has been set up and jdk21u-dev starts
collecting changes for 21.0.3.

Please let me know if there are questions.

Thanks,
Severin