[red-knot] LSP: only emit WARN logs from non-red-knot sources (#16760)
Currently the red-knot LSP server emits any log messages of level `INFO`
or higher from non-red-knot crates. This makes its output quite verbose,
because Salsa emits an `INFO` level message every time it executes a
query. I use red-knot as LSP with neovim, and this spams the log file
quite a lot.
It seems like a better default to only emit `WARN` or higher messages
from non-red-knot sources.
I confirmed that this fixes the nvim LSP log spam.