Adding metadata to server_hooks_custom_error_message_spec for clarity

What does this MR do and why?

This spec is dependant on a pre-receive hook being configured in the the test environment. We achieve that in gitlab-qa using the following script during startup - https://gitlab.com/gitlab-org/gitlab-qa/-/blob/541eac413a2116cba115f2d0f652ec6009d75db1/lib/gitlab/qa/support/config_scripts.rb

When using the :orchestrated label on it's own on a spec it doesn't actually get picked up by any job due to our current set of rules. For that reason we use the current set of labels on it, which essentially prevent it from running on any live environment but also indirectly cause it to get picked up by the jobs such as master/nightly which run the test environment in docker. It's not really obvious that this is what is happening here, nor the dependance on the dockerized environment required for this spec.
To help hint that this test only works on a dockerized environment let's add the orchestrated metadata here.

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