refactor: Update endpoint for request schema form

[programmiri]

What happened?

Currently, we're showing users a list of "schema registry environments" to choose from. They are fetched from the endpoint getEnvsForSchemaRequests.

We're now theoretically able to use "topic environments" here because the kafka envs are matched to the schema registry. This requires a backend change as well as a frontend change.

The current implementation is working well, so it's not an urgent issue.