CI use 6.1 nightlies now that Swift development is happening in the 6.1
branch

### Motivation

Fixes apple/swift-openapi-generator#730.

We were not correctly keeping `CustomStringConvertible` and
`LocalizedError` conformances separate for wrapper errors like
`ClientError` and `ServerError`. This lead to some user-thrown errors
(in handlers, transports, and middlewares) to print less information
than the error was actually providing (using a different method).

### Modifications

Properly untangle the two printing codepaths, and only call
`localizedDescription` from the wrapper error's `errorDescription`.

Also made the `localizedDescription` strings a bit more user-friendly
and less detailed, as in some apps these errors might get directly
rendered by a UI component that calls `localizedDescription`.

### Result

Error logging should now match adopter expectations.

### Test Plan

Added unit tests for `{Client,Server}Error` printing methods.

Rename nightly_6_1 params to nightly_next; see
apple/swift-nio#3122

Only apply standard swift settings on valid targets. The current check
ignores plugins but that is not comprehensive enough.