CI: fix backport validator for new branch naming

validate-backport job started to fail as only old stable branch naming
is accepted. This patch extends the script to allow numbers and dot as
well in the branch names (like stable/2023.1).

Change-Id: Icbdcd5d124717e195d55d9e42530611ed812fadd
(cherry picked from commit fe125da)

Author: Elod Illes

tools/check-cherry-picks.sh

@@ -23,7 +23,7 @@ hashes=$(git show --format='%b' --quiet $commit_hash | sed -nr 's/^.cherry picke
 checked=0
 branches+=""
 for hash in $hashes; do
-    branch=$(git branch -a --contains "$hash" 2>/dev/null| grep -oE '(master|stable/[a-z]+)')
+    branch=$(git branch -a --contains "$hash" 2>/dev/null| grep -oE '(master|stable/[a-z0-9.]+)')
     if [ $? -ne 0 ]; then
         echo "Cherry pick hash $hash not on any master or stable branches"
         exit 1