chore: Update error message to reflect tier availability

Description

We should review an error message that just says GitLab Enterprise. In !1417 (comment 1778172285) I was asked to review it. (src/desktop/gitlab/gitlab_service.ts) It's definitely a paid feature, but which tier? I'm not quite sure.

https://docs.gitlab.com/ee/user/application_security/sast/ in general is GitLab Premium and GitLab Ultimate. However, some features are available only to Ultimate. This line from the description area of !1417 (merged) makes me question if we're dealing with an Ultimate-only feature:

Connecting to a self-hosted GitLab Free or GitLab Premium should hide the security scans tree item, and show the other tree items (issue, merge request, pipeline).

I'll set up suggestions so the PM can apply whichever answer is correct, and then this MR can go to an engineer for merge.

Related Issues

• Closes docs review: Follow-up from "fix(security_scann... (#1263 - closed)
• Related to fix(security_scanning): Hide reports for GitLab... (!1417 - merged) (this MR is a follow-up change)
• Related to Unable to view pipelines due to failed security... (#1000 - closed)
• Related to Error in "For current branch" view on self-host... (#1133 - closed)
• Related to Error in the Current branch section of Gitlab w... (#1254 - closed)

How has this been tested?

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)
• Test gap