Make String::concat(const char *, unsigned int) public

From upstream ArduinoCore-API commit 3b88acac8ee00fc57bc3328e68823aa53088f891

Author: rcombs

cores/esp32/WString.h

@@ -108,6 +108,7 @@ class String {
         // concatenation is considered unsuccessful.
         unsigned char concat(const String &str);
         unsigned char concat(const char *cstr);
+        unsigned char concat(const char *cstr, unsigned int length);
         unsigned char concat(char c);
         unsigned char concat(unsigned char c);
         unsigned char concat(int num);
@@ -326,7 +327,6 @@ class String {
         void init(void);
         void invalidate(void);
         unsigned char changeBuffer(unsigned int maxStrLen);
-        unsigned char concat(const char *cstr, unsigned int length);
 
         // copy and move
         String & copy(const char *cstr, unsigned int length);