Rename default branch to main, add mirror workflow for migration (#974)

Signed-off-by: Emerson Knapp <emerson.b.knapp@gmail.com>

Author: emersonknapp

.github/workflows/automerge.yml

@@ -2,7 +2,7 @@ name: Auto merge
 
 on:
   pull_request:
-    branches: master
+    branches: [main]
   pull_request_review:
     types:
       - submitted

.github/workflows/mirror.yml

@@ -0,0 +1,15 @@
+---
+
+name: Mirror branch main to master
+
+on:
+  push:
+    branches: [main]
+
+jobs:
+  mirror-to-master:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: zofrex/mirror-branch@v1
+        with:
+          target-branch: master

.github/workflows/test.yml

@@ -2,12 +2,10 @@ name: "Test action-ros-ci"
 on:
   pull_request:
   push:
-    branches:
-      - master
-      - "releases/*"
+    branches: [main]
   schedule:
-    # Run the CI automatically twice per day to look for flakyness.
-    - cron: "0 */12 * * *"
+    # Run the CI automatically nightly to catch flakes
+    - cron: "0 0 * * *"
 
 defaults:
   run: