RFR: 8333743: Change .jcheck/conf branches property to match valid branches

[Kevin Rushforth]

Update the `branches` property in `.jcheck/conf` to allow branches, now that we are using them for JDK stabilization. This will allow integrators to use Skara to create new stabilization branches (we had to do it manually this time).

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Commit messages:
 - 8333743: Change .jcheck/conf branches property to match valid branches

Changes: https://git.openjdk.org/jdk/pull/19582/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19582&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333743
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19582.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19582/head:pull/19582

PR: https://git.openjdk.org/jdk/pull/19582