base-plugins: bump github to 1.33.1

v1.33.0 has a regression breaking GitHub webhooks. It was fixed in
v1.33.1:

jenkinsci/github-plugin#242

This is the only change in v1.33.1 so should be safe:

jenkinsci/github-plugin@v1.33.0...v1.33.1

2/contrib/openshift/base-plugins.txt

@@ -10,7 +10,7 @@ cloudbees-folder:6.15
 docker-commons:1.16
 git-client:3.2.1
 git:4.5.2
-github:1.33.0
+github:1.33.1
 google-oauth-plugin:1.0.0
 groovy:2.2
 htmlpublisher:1.21