Auto detect github target branch in rebase script (#6187)

tests/scripts/rebase.sh

 #!/bin/bash
 set -euxo pipefail
 
-# Rebase PRs on master to get latest changes
+KUBESPRAY_NEXT_VERSION=2.$(( ${KUBESPRAY_VERSION:3:2} + 1 ))
+
+# Rebase PRs on master (or release branch) to get latest changes
 if [[ $CI_COMMIT_REF_NAME == pr-* ]]; then
   git config user.email "ci@kubespray.io"
   git config user.name "CI"
+  if [[ -z "`git branch -a --list origin/release-$KUBESPRAY_NEXT_VERSION`" ]]; then
     git pull --rebase origin master
+  else
+    git pull --rebase origin release-$KUBESPRAY_NEXT_VERSION
+  fi
 fi