index: update matrix room links

The Matrix room #Go:matrix.org has been abandoned for years by lack of
admin/moderators and lots of spam.

Now the Matrix community hangs out in the
Matrix space #gophers:dendrite.matrix.org and the
Matrix room #go-lang:matrix.org.

Fixes golang/go#77721

Change-Id: Iaaa5818f647799b5f08dba4cdfe043c00f99233e
GitHub-Last-Rev: 962238b5fbf45a3eeab8af3ebefa838e8abbb88e
GitHub-Pull-Request: golang/wiki#92
Reviewed-on: https://go-review.googlesource.com/c/wiki/+/748081
Reviewed-by: Sean Liao <sean@liao.dev>