dev: update CI to use main as the default branch

.github/workflows/ci.yml

@@ -6,7 +6,7 @@ on:
   workflow_dispatch:
   push:
     branches:
-      - master
+      - main
       - v*.*.x
     tags:
       - v*.*.*

CONTRIBUTING.md

@@ -20,8 +20,8 @@ git remote add upstream https://github.com/rails/rails-html-sanitizer.git
 Make sure your fork is up-to-date and create a topic branch for your feature or bug fix.
 
 ```
-git checkout master
-git pull upstream master
+git checkout main
+git pull upstream main
 git checkout -b my-feature-branch
 ```
 
@@ -78,11 +78,11 @@ Go to https://github.com/contributor/rails-html-sanitizer and select your featur
 
 #### Rebase
 
-If you've been working on a change for a while, rebase with upstream/master.
+If you've been working on a change for a while, rebase with upstream/main.
 
 ```
 git fetch upstream
-git rebase upstream/master
+git rebase upstream/main
 git push origin my-feature-branch -f
 ```