Make channel name immutable in `darc update-channel`

[dkurepa]

Currently, the way update-channel works is that the channel id is a required parameter, and then we're allowed to change the name or the classification.

With this epic in progress and the Ingestor implementation in #5636, we noticed that we'll need to make the channel name immutable, so the update-channel subscription will no longer be able to do so (and the API endpoint).

This also makes sense business logic wise, we treat channel names as their unique IDs, at some point we were talking about even possibly removing ids and just using names as the primary keys

[adamzip]

I think that with our current logic, the channel names are already as immutable as they can be in the ingestion. Maybe we can discuss this offline?

[dkurepa]

I think we just need to update the darc update-channel command

[dkurepa]

closing this issue as it will be done as a part of a different one