New gateway onboarding

[kschiffer]

Summary

References #3947 OS counterpart of https://github.com/TheThingsIndustries/lorawan-stack/pull/3304

This PR adds the new gateway onboarding which retires the old gateway registration form. The OS version of the gateway onboarding only contains manual registration (and no claiming).

Changes

• Remove old gateway registration
• Introduce new gateway onboarding flow
• Update end-to-end tests accordingly

Testing

Manual and end-to-end tests.

Notes for Reviewers

This implementation is essentially a simplified version of https://github.com/TheThingsIndustries/lorawan-stack/pull/3304 which removed all functionality related to claiming as well as detecting claimability based on the gateway EUI.

Checklist

• [x] Scope: The referenced issue is addressed, there are no unrelated changes.
• [x] 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.
• [x] Documentation: Relevant documentation is added or updated.
• [x] Changelog: Significant features, behavior changes, deprecations and fixes are added to CHANGELOG.md.
• [x] Commits: Commit messages follow guidelines in CONTRIBUTING.md, there are no fixup commits left.