global.gitlab-ci.yml

In !105049 we added
a new Zoekt integration for code search. This MR introduced a few RSpec
tests with the `:zoekt` tag that we needed to skip in CI because they
depend on having running zoekt services. We have since created a docker
image in
gitlab-build-images!624
for running the necessary dependencies in CI as `services`. This MR
adds this image as a service and removes the rule for ignoring the
`:zoekt` tag in CI.

We also needed to update Webmock stubbing to allow requests to these
services.