try.gogs.io has been dead for several months now, and it's now having
certificate trouble.  Stop spreading a link to the suspicious website,
drop it from user-facing documentation in "Greasy Fork.md" and top-level
"README.md".

Mark it as dead with a skull and bones emoji in MirrorsAdmin.md, which
is developer-facing, similar to next.forgejo.org in commit b90b3e75
(docs: drop dead links to next.forgejo.org, 2023-12-31).  Note, that in
case of next.forgejo.org, only the _repo_ died, not the whole website.