Replace references to Amazon Root CA from GitHub OTA Example with DigiCert

[oxr463]

curl -v https://raw.githubusercontent.com
*   Trying 2606:50c0:8002::154:443...
* Connected to raw.githubusercontent.com (2606:50c0:8002::154) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
*  CAfile: /etc/ssl/certs/ca-certificates.crt
*  CApath: /etc/ssl/certs
* TLSv1.0 (OUT), TLS header, Certificate Status (22):
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
* TLSv1.2 (IN), TLS header, Certificate Status (22):
* TLSv1.3 (IN), TLS handshake, Server hello (2):
* TLSv1.2 (IN), TLS header, Finished (20):
* TLSv1.2 (IN), TLS header, Supplemental data (23):
* TLSv1.3 (IN), TLS handshake, Encrypted Extensions (8):
* TLSv1.2 (IN), TLS header, Supplemental data (23):
* TLSv1.3 (IN), TLS handshake, Certificate (11):
* TLSv1.2 (IN), TLS header, Supplemental data (23):
* TLSv1.3 (IN), TLS handshake, CERT verify (15):
* TLSv1.2 (IN), TLS header, Supplemental data (23):
* TLSv1.3 (IN), TLS handshake, Finished (20):
* TLSv1.2 (OUT), TLS header, Finished (20):
* TLSv1.3 (OUT), TLS change cipher, Change cipher spec (1):
* TLSv1.2 (OUT), TLS header, Supplemental data (23):
* TLSv1.3 (OUT), TLS handshake, Finished (20):
* SSL connection using TLSv1.3 / TLS_AES_256_GCM_SHA384
* ALPN, server accepted to use h2
* Server certificate:
*  subject: C=US; ST=California; L=San Francisco; O=GitHub, Inc.; CN=*.github.io
*  start date: Feb 21 00:00:00 2023 GMT
*  expire date: Mar 20 23:59:59 2024 GMT
*  subjectAltName: host "raw.githubusercontent.com" matched cert's "*.githubusercontent.com"
*  issuer: C=US; O=DigiCert Inc; CN=DigiCert TLS RSA SHA256 2020 CA1
*  SSL certificate verify ok.

Reference(s):

• https://github.com/arduino-libraries/Arduino_ESP32_OTA/blob/main/examples/OTA_GitHub_Server/OTA_GitHub_Server.ino#L14