Fix wrong return value of setNetworkMode and setPreferredMode

[raphac]

The example doesn't work for me. I had an endless loop. The return value of setNetworkMode and setPreferredMode is not a string, it's a bool. See:
https://github.com/vshymanskyy/TinyGSM/blob/07b402721d23e2b392251ad95cc157505bbeeb8d/src/TinyGsmClientSIM70xx.h#L213

[Mr-HaleYa]

After TinyGSM updated to 11.0.0 they changed from

LINK

  String setNetworkMode(uint8_t mode) {
    sendAT(GF("+CNMP="), mode);
    if (waitResponse(GF(GSM_NL "+CNMP:")) != 1) { return "OK"; }
    String res = stream.readStringUntil('\n');
    waitResponse();
    return res;
  }

  String setPreferredMode(uint8_t mode) {
    sendAT(GF("+CMNB="), mode);
    if (waitResponse(GF(GSM_NL "+CMNB:")) != 1) { return "OK"; }
    String res = stream.readStringUntil('\n');
    waitResponse();
    return res;
  }

to this
LINK

  bool setNetworkMode(uint8_t mode) {
    // 2 Automatic
    // 13 GSM only
    // 38 LTE only
    // 51 GSM and LTE only
    thisModem().sendAT(GF("+CNMP="), mode);
    return thisModem().waitResponse() == 1;
  }

  bool setPreferredMode(uint8_t mode) {
    // 1 CAT-M
    // 2 NB-IoT
    // 3 CAT-M and NB-IoT
    thisModem().sendAT(GF("+CMNB="), mode);
    return thisModem().waitResponse() == 1;
  }

so yes this should be approved and all users should update their TinyGSM library