serverutil/frontend: make the GitHub repos the canonical source
serverutil/frontend: make the GitHub repos the canonical source

These have been mirrors for a long time, but we will shortly move off
the upspin.googlesource.com infrastructure.

Change-Id: I767fc74091ff18893f3e3de3ec97469ca30a530c
Reviewed-on: https://upspin-review.googlesource.com/c/upspin/+/19742
Reviewed-by: 395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com <395a7d33bec8475c9b83b7d440f141bcbd994aa5@google.com>

upspin/upspin,upspin/upspin