Fix compile error

Fix deprecated functions superseded from mbed TLS v2.7.0
- mbedtls_md5_starts()
- mbedtls_md5_update() 
- mbedtls_md5_finish()

leading to following compiling error

```
/home/kh/.arduino15/packages/esp32/tools/xtensa-esp32s2-elf-gcc/gcc8_4_0-esp-2021r1/bin/../lib/gcc/xtensa-esp32s2-elf/8.4.0/../../../../xtensa-esp32s2-elf/bin/ld: libraries/ESPAsyncWebServer-master/WebAuthentication.cpp.o:(.literal._ZL6getMD5PhtPc+0x4): undefined reference to `mbedtls_md5_starts'
/home/kh/.arduino15/packages/esp32/tools/xtensa-esp32s2-elf-gcc/gcc8_4_0-esp-2021r1/bin/../lib/gcc/xtensa-esp32s2-elf/8.4.0/../../../../xtensa-esp32s2-elf/bin/ld: libraries/ESPAsyncWebServer-master/WebAuthentication.cpp.o: in function `getMD5(unsigned char*, unsigned short, char*)':
/home/kh/Arduino/libraries/ESPAsyncWebServer-master/src/WebAuthentication.cpp:73: undefined reference to `mbedtls_md5_starts'
collect2: error: ld returned 1 exit status
exit status 1
Error compiling for board ESP32S2 Dev Module.
```

Author: khoih-prog

src/WebAuthentication.cpp

@@ -22,6 +22,7 @@
 #include <libb64/cencode.h>
 #ifdef ESP32
 #include "mbedtls/md5.h"
+#include "mbedtls/version.h"
 #else
 #include "md5.h"
 #endif
@@ -71,9 +72,16 @@ static bool getMD5(uint8_t * data, uint16_t len, char * output){//33 bytes or mo
   memset(_buf, 0x00, 16);
 #ifdef ESP32
   mbedtls_md5_init(&_ctx);
-  mbedtls_md5_starts(&_ctx);
-  mbedtls_md5_update(&_ctx, data, len);
-  mbedtls_md5_finish(&_ctx, _buf);
+  #if (MBEDTLS_VERSION_NUMBER < 0x02070000)
+    // Superseded from v2.7.0
+    mbedtls_md5_starts(&_ctx);
+    mbedtls_md5_update(&_ctx, data, len);
+    mbedtls_md5_finish(&_ctx, _buf);
+  #else
+    mbedtls_md5_starts_ret(&_ctx);
+    mbedtls_md5_update_ret(&_ctx, data, len);
+    mbedtls_md5_finish_ret(&_ctx, _buf);
+  #endif
 #else
   MD5Init(&_ctx);
   MD5Update(&_ctx, data, len);