cli: Suggest to make new identity the default for server (#1109)

When a server key rotation is suspected, suggest to make the new
identity the default.

This usually happens during development using an ephemeral instance. If
one follows the instructions, the `default_identity` is not set for the
existing server, which makes the CLI generate a fresh identity every
time.

See also: #333

crates/cli/src/util.rs

@@ -361,7 +361,7 @@ Has the server rotated its keys?
 Remove the outdated identity with:
 \tspacetime identity remove {identity}
 Generate a new identity with:
-\tspacetime identity new --no-email --server {server}"
+\tspacetime identity new --no-email --server {server} --default"
         )
     })
 }