Remove fleets from gateway onboarding and add link

[ryaplots]

Summary

As part of the gateway fleet restructure, this PR removes fleets from the stack and adds links to new status page.

Changes

• Remove fleets logic
• Add gateway status link

Testing

Steps

1. Try to claim a TTIG PRO that is not connected to a fleet.
2. See the error and click on the link in the error.

Checklist

• Scope: The referenced issue is addressed, there are no unrelated changes.
• Compatibility: The changes are backwards compatible with existing API, storage, configuration and CLI, according to the compatibility commitments in README.md for the chosen target branch.
• Documentation: Relevant documentation is added or updated.
• Testing: The steps/process to test this feature are clearly explained including testing for regressions.
• Infrastructure: If infrastructural changes (e.g., new RPC, configuration) are needed, a separate issue is created in the infrastructural repositories.
• Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.