Update ghprbActualCommit to default to main for clean up job

We have recently deleted master branch from metal3-io/project-infra
repo. And our clean up job runs from project-infra. Since the default
and the only branch that exists is main, we need to update defaulting
here too since this is from where the value is passed to Jenkins
pipeline.

Change-Id: Ib9bbbcd69a47c8bf50bdae2cb2e59de60f6f7e3b
diff --git a/jjb/airship/job_master_clean.yml b/jjb/airship/job_master_clean.yml
index 99de2d0..43cd780 100644
--- a/jjb/airship/job_master_clean.yml
+++ b/jjb/airship/job_master_clean.yml
@@ -44,7 +44,7 @@
         description: 'Project github repo name'
     - string:
         name: ghprbActualCommit
-        default: 'master'
+        default: 'main'
         description: 'CI Repo branch for triggering manual build'
     - string:
         name: ghprbAuthorRepoGitUrl