RFR: 2068: Backport command should use default branch of repo as default

[Zhao Song]

When a user is using /backport command to create a backport and the user doesn't specify a target branch, skara bot would use "master" as a default target branch. However, in some repos, like lilliput-jdk17u and lilliput-jdk21u, the default branch is not "master". Therefore, it will be better to use the repository's default branch as the default target branch of /backport command.

As Erik said, the value of default branch is already available in the repository JSON. So we just need to implement a `getDefaultBranchName` method.

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

Commit messages:
 - SKARA-2068

Changes: https://git.openjdk.org/skara/pull/1572/files
 Webrev: https://webrevs.openjdk.org/?repo=skara&pr=1572&range=00
  Issue: https://bugs.openjdk.org/browse/SKARA-2068
  Stats: 46 lines in 9 files changed: 45 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/skara/pull/1572.diff
  Fetch: git fetch https://git.openjdk.org/skara.git pull/1572/head:pull/1572

PR: https://git.openjdk.org/skara/pull/1572