chore(syncthing): remove support for TLS 1.2 sync connections (#10064)

This cleans up the option to allow old TLS 1.2 sync connections. The
flag existed for compatibility with old Syncthing versions that don't
support TLS 1.3, which is approximately Syncthing 1.2.2 (September 2019)
and older. ("Approximately" because it depends on the Go version it's
built with and that's when we switched to building with Go 1.13.)

Ref #10062 because it reminded me this exists.

lib/syncthing/syncthing.go

@@ -252,13 +252,7 @@ func (a *App) startup() error {
 	// The TLS configuration is used for both the listening socket and outgoing
 	// connections.
 
-	var tlsCfg *tls.Config
-	if a.cfg.Options().InsecureAllowOldTLSVersions {
-		l.Infoln("TLS 1.2 is allowed on sync connections. This is less than optimally secure.")
-		tlsCfg = tlsutil.SecureDefaultWithTLS12()
-	} else {
-		tlsCfg = tlsutil.SecureDefaultTLS13()
-	}
+	tlsCfg := tlsutil.SecureDefaultTLS13()
 	tlsCfg.Certificates = []tls.Certificate{a.cert}
 	tlsCfg.NextProtos = []string{bepProtocolName}
 	tlsCfg.ClientAuth = tls.RequestClientCert