New Repository
Git will use a user-defined template when starting a new git repository, so configure that template to use "main" instead of the default.
git config --global init.templateDir '~/.git-template'
cp -r /usr/local/share/git-core/templates ~/.git-template
echo 'ref: refs/heads/main' > ~/.git-template/HEAD
From that point on git init will use the template with the HEAD pointing to main.
Existing Repository