feat: Disable SNI for STARTTLS (#7499)

Many clients don't send it currently, so it is unlikely that servers depend on it:
https://mastodon.social/@cks/114690055923939576.

For "implicit TLS", do not turn it off yet, it will serve as a fallback in case of rare server that
needs it. If the server only supports STARTTLS and requires SNI then it is really weird, likely
should not happen.

src/net/proxy.rs

@@ -429,9 +429,11 @@ impl ProxyConfig {
                     load_cache,
                 )
                 .await?;
+                let use_sni = true;
                 let tls_stream = wrap_rustls(
                     &https_config.host,
                     https_config.port,
+                    use_sni,
                     "",
                     tcp_stream,
                     &context.tls_session_store,