Deprecate and remove support for unused '/ci' in GitLab's URL

/ci in URL path is no longer used by Runner and GitLab. With 9.0 we've added a mechanism to fix URLs that were added to config.toml file in the past, when the path was needed.

However, at some point we should remove support for the old URLs. This forces users to update their configuration, so such change should be announced and deprecated at some point, and removed in one of next releases. This will give everyone possibility to update configuration files.