utils/shared_audits: allow subgroups in gitlab url

[cho-m]

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To fix: https://github.com/Homebrew/homebrew-core/actions/runs/10570029877/job/29283904401#step:3:38

Subgroups: https://docs.gitlab.com/ee/user/group/subgroups/

Extra characters: https://docs.gitlab.com/ee/user/reserved_names.html#limitations-on-usernames-project-and-group-names-and-slugs

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The starting \w is a slightly shorter-to-type superset of [A-Za-z0-9] (#{Alnum}). Mainly to avoid [\w.-]+ matching unique /-/; however, not really needed as there should never be a URL with /-/-/

[MikeMcQuaid]

Thanks again @cho-m!