www/forgejo: Switch to using "FORGEJO_" prefixed environment variables

Switch the environment variables specified in the Forgejo RC file from
"GITEA_" to "FORGEJO_".
The old variables are still work, but new are preffered:
https://codeberg.org/forgejo/forgejo/src/commit/7015bdfa480e717299377b4547320ae6d70e9da2/modules/setting/path.go#L123
Also, the Forgejo documentation uses new names:
https://forgejo.org/docs/latest/admin/config-cheat-sheet/

PR: 282771
Approved by: Stefan Bethke <stb@lassitu.de> (maintainer)