Rename `master` branch to `main` (#914)

.github/workflows/codecov.yml

@@ -2,7 +2,7 @@ name: Coverage
 
 on:
   push:
-    branches: ["master"]
+    branches: ["main"]
   pull_request:
 
 jobs:

.github/workflows/main.yml

@@ -2,7 +2,7 @@ name: CI
 
 on:
   push:
-    branches: ["master"]
+    branches: ["main"]
   pull_request:
   merge_group:
     types: [checks_requested]