github: Remove reference to master branch.

The OvS primary development branch has been renamed main
so there is no longer any need for this CI configuration
to refer to master.

Signed-off-by: Simon Horman <horms@ovn.org>

.github/workflows/build-and-test.yml

@@ -307,7 +307,7 @@ jobs:
             MIN_DISTANCE=1000
             git remote add upstream https://github.com/openvswitch/ovs.git
             git fetch upstream
-            for upstream_head in $(git ls-remote --heads upstream main master dpdk-latest branch-2.17 branch-[3456789]* | cut -f 1); do
+            for upstream_head in $(git ls-remote --heads upstream main dpdk-latest branch-2.17 branch-[3456789]* | cut -f 1); do
               CURR_BASE=$(git merge-base ${upstream_head} HEAD 2>/dev/null)
               if [ ${CURR_BASE} ]; then
                 DISTANCE=$(git log --oneline ${CURR_BASE}..HEAD | wc -l);