Update skaraMirror.sh to handle upstream jdk(head) version branching

[andrew-m-leonard]

Fixes adoptium/temurin-build#3841

Fixes the Adoptium mirror to handle upstream OpenJDK version branches.

• Now the BRANCH environmental variable can be set to a non-master branch, eg.jdk23
• This causes the mirror to mirror the upstream jdk23 branch to the same within the mirror, and also merge into dev and release branches called:
  • dev_$BRANCH
  • release_$BRANCH

The behaviour after this change, is not changed if BRANCH is not set or is value "master".

After this change is merged, the jdk(head) mirror Jenkins job shell script will be updated to the following:

export releaseTagExcludeList=""

branches="master jdk23"

for branch in $branches
do
  export BRANCH="$branch"
  bash ./skaraMirror.sh jdk
done

[andrew-m-leonard]

Tested on my mirror "fork", and test build of jdk-23 from it is successful.
Also, file compare with upstream repo successful.