Proposal: Use backports for release stabilization

[Mark Reinhold]

Last month, Erik Duveblad proposed that we change the way in which we
stabilize releases [1].

To summarize: Currently, to fix a bug in the release being stabilized
you create a pull request against the stabilization repository (e.g.,
jdk21).  Jesper periodically merges changes from the stabilization
repository to the main-line repository (jdk), but if the merge contains
commits that were intended only for the stabilization repository then
they have to be backed out -- which sometimes doesn’t happen.

Erik proposes that we instead use the Skara backport mechanism, which
we already use for update releases.  To fix a bug in the release being
stabilized you’d first create a PR against the main-line repository.
Then, after that PR is reviewed and integrated, you’d use Skara’s
`/backport` command to backport the fix to the stabilization repository.
If your fix is intended only for the stabilization repository then you’d
create a PR directly against that repository.

Please see Erik’s proposal [1] for further details.

In my view this is a good proposal, and I thank Erik for writing it up.
We’ve already had some discussion of it, and Erik and Kevin addressed
the concerns raised.  Further feedback from JDK Project Committers and
Reviewers [2] is welcome, as are reasoned objections.  If no such
objections are raised by 23:59 UTC on Monday, 29 May, or if they’re
raised and then satisfactorily answered, then we’ll adopt this process
for JDK 21 and also update JEP 3 [3] and the Developers’ Guide [4]
accordingly.

- Mark


[1] https://mail.openjdk.org/pipermail/jdk-dev/2023-April/007612.html
[2] https://openjdk.org/census#jdk
[3] https://openjdk.org/jeps/3
[4] https://openjdk.org/guide/