devops: rename default branch to main (#1027)

Author: mxschmitt

.github/workflows/ci.yml

@@ -3,11 +3,11 @@ name: CI
 on:
   push:
     branches:
-      - master
+      - main
       - release-*
   pull_request:
     branches:
-      - master
+      - main
       - release-*
 
 jobs:

CONTRIBUTING.md

@@ -47,7 +47,7 @@ pre-commit install
 pre-commit run --all-files
 ```
 
-For more details look at the [CI configuration](./blob/master/.github/workflows/ci.yml).
+For more details look at the [CI configuration](./blob/main/.github/workflows/ci.yml).
 
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