account for the 8.x in DRA publishing task (#16436)

the current DRA publishing task computes the branch from the version
contained in the version.yml

This is done by taking the major.minor and confirming that a branch
exists with that name.

However this pattern won't be applicable for 8.x, as that branch
currently points to 8.16.0 and there is no 8.16 branch.

This commit falls back to reading the buildkite injected
BUILDKITE_BRANCH variable.

Author: jsvd

.buildkite/scripts/dra/publish.sh

@@ -15,8 +15,9 @@ RELEASE_VER=`cat versions.yml | sed -n 's/^logstash\:[[:space:]]\([[:digit:]]*\.
 if [ -n "$(git ls-remote --heads origin $RELEASE_VER)" ] ; then
     RELEASE_BRANCH=$RELEASE_VER
 else
-    RELEASE_BRANCH=main
+    RELEASE_BRANCH="${BUILDKITE_BRANCH:="main"}"
 fi
+echo "RELEASE BRANCH: $RELEASE_BRANCH"
 
 if [ -n "$VERSION_QUALIFIER_OPT" ]; then
   # Qualifier is passed from CI as optional field and specify the version postfix