Update backport.yml for v/5.5 (#1178)

5.5.0 branch has been created so adding v/5.5 backport branch

As part of
https://hazelcast.atlassian.net/wiki/spaces/EN/pages/4681138242/Create+branch+5.x#Update-hz-docs-backport.yml

.github/workflows/backport.yml

@@ -7,7 +7,7 @@ jobs:
   backport:
     strategy:
       matrix:
-        branch: ['v/5.0', 'v/5.1', 'v/5.2', 'v/5.3', 'v/5.4']
+        branch: ['v/5.0', 'v/5.1', 'v/5.2', 'v/5.3', 'v/5.4', 'v/5.5']
     runs-on: ubuntu-latest
     steps: