Add gl_service label to infrastructure labels and tags

Why is this change being made?

Add gl_service as an optional label that links resources to entries in the GitLab service catalog for ownership and operational responsibility.

This label supports cost allocation and incident management by establishing clear service ownership. The label is optional and should reference valid service catalog entries.

This is an additive change. We make this change without throwing everything out in the labeling standard, with the intention to learn from labeling gitlab-production resources and only then adjust the broadly scoped standard.

Preceding work

1. GitLab Production Labeling Strategy (!17452 - closed) - original idea to target gitlab-production only, but we talked about it with a broader scope in mind (non-conclusively).
2. We decided to start adding the 4 labels we prominently talk about to gitlab-production to get started and learn from that
3. Update Infrastructure Labels and Tags Handbook ... (!17800 - closed) - targeted to replace the entire labeling standard for all departments with only a few labels, concerns about scope and rigidity

Given the above context, the intention here is to enable https://gitlab.com/groups/gitlab-com/gl-infra/finops/-/work_items/101+, so we can start adding these 4 labels to gitlab-production (the GCP project) only:

1. gl_product_category ☑️ included in the existing standard
2. gl_service ➕ Added to the standard (this change)
3. gl_env_type ☑️ included in the existing standard ❗ values are different in reality (gprd vs prd) - we likely can't change this to prd easily, stick with gprd and apply throughout the entire gitlab-production project
4. gl_resource_type ☑️ included in the existing standard

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