prepare for renaming master to main (#1174)

Prepare this repo for switching its main branch name to `main` by
making the workflows run on both `master` and `main`.  Some workflows
already run that way, but make sure all of them do, ahead of going
ahead and renaming it.

Once the branch is renamed, we can drop the build on `master` here.

.github/workflows/codeql.yml

@@ -2,9 +2,9 @@ name: "CodeQL"
 
 on:
   push:
-    branches: [ "master" ]
+    branches: [ "master", "main" ]
   pull_request:
-    branches: [ "master" ]
+    branches: [ "master", "main" ]
   schedule:
     - cron: '31 7 * * 6'
 

README.md

@@ -13,7 +13,7 @@ name: Pulumi
 on:
   push:
     branches:
-      - master
+      - main
 jobs:
   up:
     name: Preview