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

[Kevin Rushforth]

Clean backport of `.jcheck/conf` change to jdk23.

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Commit messages:
 - Backport 2a37764e7428d579a3080e62681f1c9c9f816c1e

Changes: https://git.openjdk.org/jdk/pull/19584/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19584&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/19584.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19584/head:pull/19584

PR: https://git.openjdk.org/jdk/pull/19584