jfx21u repo created, but not yet generally open for backports

[Kevin Rushforth]

Skip this unless you care about update release repositories (I expect 
most developer will not care).

You may have noticed that a jfx20u [1] repo was created, matching what 
the JDK had already done. Prior to this release, e.g., for the 19.0.X 
update releases, we were using a rather ad hoc model for non-LTS update 
releases that differed from that of LTS releases. For LTS releases, we 
have a separate updates repository for each code line, for example, 
jdk17u [2] and jdk11u [3]. For non-LTS releases, we had previously 
continued to use the feature release stabilization branch for subsequent 
update releases following the GA of the feature release. That has a 
couple of drawbacks. First and foremost, it means we have to remember to 
ask the question "is this an LTS release or not" to know whether to use 
the `jfxNN` branch in the mainline repo or the `master` branch in the 
`jfxNNu` repo. The second drawback is that with the previous model, 
there is no place to put fixes for the .0.1 update release until the 
feature release ships.

In order to unify the procedures for LTS and non-LTS releases, I plan to 
always create a `jfxNNu` repo for each release. The natural time to 
create this would be some time during RDP1 of the feature release (a 
week or two after the `jfxNN` branch is forked in the main repo). By 
this measure, the jfx20u repo is already several weeks late in being 
created, but it's still a little earlier than the jfx20 branch would 
have been available for 20.0.1 fixes.

This will not impact anything related to stabilization of feature 
releases, which will continue to use stabilization branches, such as 
jfx20, in the main jfx repo.

NOTE: Since this is the first such release after transitioning to the 
new repo model, and because we are already very late in the cycle for 
the April CPU, we do not plan to accept unsolicited backport PRs to 
jfx20u for 20.0.1.

-- Kevin

[1] https://github.com/openjdk/jfx20u
[2] https://github.com/openjdk/jfx17u
[3] https://github.com/openjdk/jfx11u