Update workflows to trigger for main (#15310)

Preparation for #13985 to make the transition from `master` to `main` as
seamless as possible.
This PR can be merged as is preferably before the branch is renamed to
avoid issues.

Author: cdce8p

.github/workflows/build_wheels.yml

@@ -2,7 +2,7 @@ name: Trigger wheel build
 
 on:
   push:
-    branches: [master, 'release*']
+    branches: [main, master, 'release*']
     tags: ['*']
 
 jobs:

.github/workflows/docs.yml

@@ -3,7 +3,7 @@ name: Check documentation build
 on:
   workflow_dispatch:
   push:
-    branches: [master, 'release*']
+    branches: [main, master, 'release*']
     tags: ['*']
   pull_request:
     paths:

.github/workflows/test.yml

@@ -3,7 +3,7 @@ name: Tests
 on:
   workflow_dispatch:
   push:
-    branches: [master, 'release*']
+    branches: [main, master, 'release*']
     tags: ['*']
   pull_request:
     paths-ignore:

.github/workflows/test_stubgenc.yml

@@ -3,7 +3,7 @@ name: Test stubgenc on pybind11-mypy-demo
 on:
   workflow_dispatch:
   push:
-    branches: [master, 'release*']
+    branches: [main, master, 'release*']
     tags: ['*']
   pull_request:
     paths: