This answer has 2 parts:
This was announced 2 months ago: Preparing for the v6 release.
The branches named development-v6 won't be updated anymore.
Now you need to use development branches to run Pi-hole v6.
Yes, but using development.
pihole checkout dev won't use branch dev. It will use development branches.
This command is a Shortcut equivalent to these 3 commands in sequence:
pihole checkout core development
pihole checkout web development
pihole checkout ftl development