[OTA] Add Github Digicert cert hash check (#1965)

to avoid loading a deprecated certificate

main/certs/default_ota_cert.h

@@ -1,5 +1,5 @@
 // The certificate must be in PEM ascii format.
-// The default certificate is for github.
+// The default certificate is for ota.openmqttgateway.com
 const char* OTAserver_cert PROGMEM = R"EOF("
 -----BEGIN CERTIFICATE-----
 MIIDQTCCAimgAwIBAgITBmyfz5m/jAo54vB4ikPmljZbyjANBgkqhkiG9w0BAQsF