Remove `Gateways` API from linkerd-viz

[kleimkuhler]

What problem are you trying to solve?

The Gateways API was deprecated as part of 2.12 (#8600) and should be fully removed in 2.13.

How should the problem be solved?

The API implementation should be removed.

Any alternatives you've considered?

N/A

How would users interact with this feature?

No response

Would you like to work on this feature?

No response

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 14 days if no further activity occurs. Thank you for your contributions.

[alpeb]

From what I understand, the usage of this API was removed from the CLI, but it's still being used by the web component to display the Gateways section of the dashboard...