Here’s the docs on changing what branch your environments link to:
In my quick testing, I’m able to delete branch master without an error (with or without fiddling with the environments)–but only seemingly. When I refresh the browser tab, the master branch reappears.
As far as I can tell, you’ll be able to use main and just ignore the vestigial master branch. But the “Merge changes into master” button will still be there.
(By the way, welcome to the forum!)