test: use tls 1.2 on TLSClientAuth test (elastic#11510)

This test is failing on Go 1.21+ because the alert/error message was
fixed is now returning 'certificate required' instead of
'bad certificate'.

See https://go.dev/doc/go1.21#crypto/tls

systemtest/tls_test.go

@@ -94,7 +94,11 @@ func TestTLSClientAuth(t *testing.T) {
 		return nil
 	}
 
-	err := attemptRequest(t, &tls.Config{InsecureSkipVerify: true})
+	err := attemptRequest(t, &tls.Config{
+		InsecureSkipVerify: true,
+		// TODO remove this and update the error message once we bump to go 1.21
+		MaxVersion: tls.VersionTLS12,
+	})
 	require.Error(t, err)
 	assert.Regexp(t, "tls: bad certificate", err.Error())
 	logs := srv.Logs.Iterator()