feat: use Ed25519 keys for sync connections (#10162)

This updates our key generation to use Ed25519 keys/certificates for
sync connections. Certificates for browser use remain ECDSA for wider
compatibility.

Ed25519 is more modern and has fewer concerns for the future than the
ECDSA curves we used previously. It is supported from Go 1.13 and
forwards, which is Syncthing 1.3.0 (October 2019).

cmd/infra/strelaypoolsrv/main.go

@@ -620,7 +620,7 @@ func createTestCertificate() tls.Certificate {
 	}
 
 	certFile, keyFile := filepath.Join(tmpDir, "cert.pem"), filepath.Join(tmpDir, "key.pem")
-	cert, err := tlsutil.NewCertificate(certFile, keyFile, "relaypoolsrv", 20*365)
+	cert, err := tlsutil.NewCertificate(certFile, keyFile, "relaypoolsrv", 20*365, false)
 	if err != nil {
 		log.Fatalln("Failed to create test X509 key pair:", err)
 	}