all: Deprecate TLS 1.2 on sync connections (fixes #7594) (#7598)

This makes us use TLS 1.3+ on sync connections by default. A new option
`insecureAllowOldTLSVersions` exists to allow communication with TLS
1.2-only clients (roughly Syncthing 1.2.2 and older). Even with that
option set you get a slightly simplified setup, with the cipher suite
order fixed instead of auto detected.

lib/tlsutil/tlsutil_test.go

@@ -98,7 +98,7 @@ func TestCheckCipherSuites(t *testing.T) {
 		tls.TLS_RSA_WITH_AES_256_CBC_SHA,
 	}
 
-	suites := buildCipherSuites()
+	suites := SecureDefaultWithTLS12().CipherSuites
 
 	if len(suites) != len(allSuites) {
 		t.Fatal("should get a list representing all suites")