security/py-certbot-dns-gandi: adjust PORTNAME

The PORTNAME for this plugin did not align with other DNS
plugins for certbot. Switch the PORTNAME around, and then
set GH_PROJECT to match the old name. (And this happened
because I dropped the PKGNAME line when portlint complained
about it)

PR: 266860
Reported by: Boudhayan Gupta (maintainer)