proxy: pg17 fixes (#8321)

## Problem

#7809 - we do not support sslnegotiation=direct
#7810 - we do not support negotiating down the protocol extensions.

## Summary of changes

1. Same as postgres, check the first startup packet byte for tls header
`0x16`, and check the ALPN.
2. Tell clients using protocol >3.0 to downgrade

proxy/src/bin/pg_sni_router.rs

@@ -216,10 +216,11 @@ async fn ssl_handshake<S: AsyncRead + AsyncWrite + Unpin>(
     use pq_proto::FeStartupPacket::*;
 
     match msg {
-        SslRequest => {
+        SslRequest { direct: false } => {
             stream
                 .write_message(&pq_proto::BeMessage::EncryptionResponse(true))
                 .await?;
+
             // Upgrade raw stream into a secure TLS-backed stream.
             // NOTE: We've consumed `tls`; this fact will be used later.