ws: use the same type for the schema keys array

Plus, it's not really a constexpr in this context

code/espurna/led.cpp

@@ -320,7 +320,7 @@ void _ledWebSocketOnConnected(JsonObject& root) {
     JsonObject& config = root.createNestedObject("ledConfig");
 
     {
-        static constexpr const char* const schema_keys[] PROGMEM = {
+        static const char* const schema_keys[] PROGMEM = {
             "ledGpio",
             "ledInv",
             "ledMode"

code/espurna/relay.cpp

@@ -1225,7 +1225,7 @@ void _relayWebSocketSendRelays(JsonObject& root) {
     config["start"] = 0;
 
     {
-        static constexpr const char* const schema_keys[] PROGMEM = {
+        static const char* const schema_keys[] PROGMEM = {
             "relayProv",
             "relayName",
             "relayBoot",

code/espurna/scheduler.cpp

@@ -159,7 +159,7 @@ void _schWebSocketOnConnected(JsonObject &root){
     config["max"] = scheduler::build::max();
 
     {
-        static constexpr const char* const schema_keys[] PROGMEM = {
+        static const char* const schema_keys[] PROGMEM = {
             "schEnabled",
             "schRestore",
             "schType",

code/espurna/wifi.cpp

@@ -1903,7 +1903,7 @@ void onConnected(JsonObject& root) {
     root["max"] = wifi::build::NetworksMax;
 
     {
-        const char* schema_keys[] = {
+        static const char* const schema_keys[] PROGMEM = {
             "ssid",
             "pass",
             "ip",