New branch and feature freeze for Lucene 10.3.0

NOTICE:

Branch branch_10_3 has been cut and versions updated to 10.4 on stable
branch.

Please observe the normal rules:

* No new features may be committed to the branch.
* Documentation patches, build patches and serious bug fixes may be
  committed to the branch. However, you should submit all patches you
  want to commit as pull requests first to give others the chance to review
  and possibly vote against them. Keep in mind that it is our
  main intention to keep the branch as stable as possible.
* All patches that are intended for the branch should first be committed
  to the unstable branch, merged into the stable branch, and then into
  the current release branch.
* Normal unstable and stable branch development may continue as usual.
  However, if you plan to commit a big change to the unstable branch
  while the branch feature freeze is in effect, think twice: can't the
  addition wait a couple more days? Merges of bug fixes into the branch
  may become more difficult.
* Only Github issues with Milestone 10.3
  will delay a release candidate build.
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