Backport of 'Fix self-managed admin/owner cannot enroll namespaces into Orbit'

What does this MR do and why?

Backport of !246338 (merged).

Patch application: the net diff from !246338 (merged) did not apply cleanly to 19-0-stable-ee. Adaptations:

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1. ee/app/policies/ee/group_policy.rb - the policy change itself (removing the knowledge_graph_saas_mode condition and the SaaS gate from the prevent rule) is semantically identical, but the surrounding code differs at 19.0 (the knowledge_graph_namespace_eligible condition uses licensed_feature_available?(:orbit) directly instead of the OrbitLicense.namespace_enrollable_or_enrolled? helper added later). Applied manually at the correct line offsets.
2. ee/app/assets/javascripts/orbit/components/orbit_explore_empty_state.vue - on 19.0, the i18n string is defined in a const i18n object at the top of the script section (line 16), not inline in the template. Changed the string in the i18n object.
3. ee/spec/frontend/orbit/components/orbit_explore_empty_state_spec.js - this file does not exist on 19-0-stable-ee. Skipped (the vue component string change is still covered by the i18n object edit).
4. ee/spec/policies/ee/group_policy_spec.rb - the 19.0 spec lacks the orbit_enroll_namespace feature flag contexts that exist on master. Adapted the spec to the 19.0 structure: split the existing single context into "when on self-managed" and "when on SaaS" branches, with admin/owner/maintainer/guest assertions matching the master MR's intent. The "when on SaaS" context retains the original assertions; the "when on self-managed" context adds admin-mode and owner-allowed assertions.
5. ee/spec/requests/api/graphql/group/knowledge_graph_spec.rb - applied cleanly (identical context lines at 19.0).
6. locale/gitlab.pot - line offsets differ. Applied manually (added the new string, removed the old one).

Related to #606374 (closed)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
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• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, request help in the #s_developer_experience Slack channel to confirm the failures are unrelated to the merge request.

Note to the merge request author and maintainer

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