Rename master to main in release script

Signed-off-by: Matt Stratton <matt.stratton@hey.com>

utils/release-theme.sh

@@ -43,8 +43,8 @@ case $version_level in
 esac
 
 git stash
-git checkout master
-git pull origin master
+git checkout main
+git pull origin main
 github_changelog_generator --user mattstratton --project castanet --future-release "$new_version" --no-issues-wo-labels --no-pr-wo-labels --no-compare-link --exclude-labels='duplicate,question,invalid,wontfix,do-not-merge,no-changelog'
 sed -i '' "s/$current_version/$new_version/g" ./theme.toml