Revert to '3.x' rather than 'main' to unbreak the release dashboard (#710)

Author: zware

master/custom/branches.py

@@ -22,12 +22,12 @@
 
 def generate_branches():
     yield BranchInfo(
-        'main',
+        '3.x',
         version_tuple=(3, 15),
         git_branch='main',
         is_main=True,
-        builddir_name='main',
-        builder_tag='main',
+        builddir_name='3.x',
+        builder_tag='3.x',
         sort_key=-9999,
     )
     yield _maintenance_branch(3, 14)