Update greenplum-database-release default repo to main

The repo https://github.com/greenplum-db/greenplum-database-release has
been changed default repo from master to main. This is to sync up this
change
Co-authored-by: NNing Wu <ningw@vmware.com>
Co-authored-by: NShaoqi Bai <bshaoqi@vmware.com>

concourse/pipelines/gpdb_master-generated.yml

@@ -12,7 +12,7 @@
 ## file (example: templates/gpdb-tpl.yml) and regenerate the pipeline
 ## using appropriate tool (example: gen_pipeline.py -t prod).
 ## ----------------------------------------------------------------------
-## Generated by gen_pipeline.py at: 2020-10-01 13:46:54.654673
+## Generated by gen_pipeline.py at: 2020-11-02 11:53:06.131787
 ## Template file: gpdb-tpl.yml
 ## OS Types: ['centos6', 'centos7', 'ubuntu18.04', 'win']
 ## Test Sections: ['ICW', 'Replication', 'ResourceGroups', 'Interconnect', 'CLI', 'UD', 'Extensions']
@@ -517,7 +517,7 @@ resources:
   type: git
   icon: git
   source:
-    branch: master
+    branch: main
     uri: https://github.com/greenplum-db/greenplum-database-release.git
 
 - name: license_file

concourse/pipelines/templates/gpdb-tpl.yml

@@ -585,7 +585,7 @@ resources:
   type: git
   icon: git
   source:
-    branch: master
+    branch: main
     uri: https://github.com/greenplum-db/greenplum-database-release.git
 
 - name: license_file