Change `master` branch to `main` branch (#20290)

config/deploy.rb

@@ -3,7 +3,7 @@
 lock '3.17.1'
 
 set :repo_url, ENV.fetch('REPO', 'https://github.com/mastodon/mastodon.git')
-set :branch, ENV.fetch('BRANCH', 'master')
+set :branch, ENV.fetch('BRANCH', 'main')
 
 set :application, 'mastodon'
 set :rbenv_type, :user