Repos: master -> main

[flatcap]

I've renamed the default branch in a bunch of NeoMutt repos.
Most of them you'll never have heard of :-)

I haven't touched the "important" repos -- they'll take a bit more thought
(and adjustment to automation)

https://github.com/neomutt/aur-build
https://github.com/neomutt/coccinelle
https://github.com/neomutt/community
https://github.com/neomutt/docbook
https://github.com/neomutt/flatcap-brain-dump
https://github.com/neomutt/flatcap-vim
https://github.com/neomutt/gentoo-neomutt
https://github.com/neomutt/gfx
https://github.com/neomutt/homebrew-core
https://github.com/neomutt/lua-samples
https://github.com/neomutt/management
https://github.com/neomutt/muttrc-mode-el
https://github.com/neomutt/neo-bot
https://github.com/neomutt/neomutt-contrib
https://github.com/neomutt/neomutt-docs
https://github.com/neomutt/neomutt-test-files
https://github.com/neomutt/neomutt.vim
https://github.com/neomutt/panel-manager
https://github.com/neomutt/release
https://github.com/neomutt/sample-data
https://github.com/neomutt/sample-mail
https://github.com/neomutt/samples
https://github.com/neomutt/sidebar-history
https://github.com/neomutt/travis-build

Cheers,
Rich / FlatCap