RFR: 2107: Change naming of branches created by /backport command

[Zhao Song]

Since in [JDK-8320358](https://bugs.openjdk.org/browse/JDK-8320358), the GitHub action would ignore the branches whose name start with 'jdk', we should change the naming of branches created by backport command.

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Commit messages:
 - SKARA-2107

Changes: https://git.openjdk.org/skara/pull/1588/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1588&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2107
  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1588.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1588/head:pull/1588

PR: https://git.openjdk.org/skara/pull/1588