RFR: 8321123: [Shenandoah/JDK21] Fix repo permissions

[Roman Kennke]

The Shenandoah/JDK21u repo has the wrong permissons: it inherits permissions from jdk-updates, but should use permission from she Shenandoah project instead.

-------------

Commit messages:
 - 8321123: [Shenandoah/JDK21] Fix repo permissions

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/3/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=3&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321123
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/3.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/3/head:pull/3

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/3