Remove namespace from GatewayClass

What does this MR do?

Remove namespace from GatewayClass

The rendered GatewayClass carried a metadata.namespace set to the release namespace. GatewayClass is cluster-scoped, so the field has no meaning there.

Installing through the Helm CLI was unaffected but other clients may fail handling this manifest. This was reported with the GitLab Operator.

Relates https://gitlab.com/gitlab-com/request-for-help/-/work_items/5148

Changelog: fixed

Related issues

Author checklist

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Required

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• Merge Request Title and Description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• MR has a green pipeline.
• Documentation created/updated.
• Tests added/updated.
• Have you validated that your change works end-to-end on a real cluster deployment?
  • Describe how you did it in a comment.
• Equivalent MR/issue for omnibus-gitlab opened.

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• MR has a green pipeline on https://gitlab.com/gitlab-org/charts/gitlab.
• Consider downstream impact to the Operator, as per evaluating impact from changes to GitLab chart.