[tor-dev] goptlib moved to gitlab.torproject.org

I have moved the goptlib repository from git.torproject.org to gitlab.torproject.org.

If you want to use it at the new location, change this import: import "git.torproject.org/pluggable-transports/goptlib.git" to this: import "gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/goptlib" and then run `go get` and `go mod tidy`.

Here is the issue about moving anti-censorship team projects from git.torproject.org to gitlab.torproject.org generally: https://bugs.torproject.org/tpo/anti-censorship/team/86#note_2823122

The old git.torproject.org URL should continue to work for the time being.