Switch From Letsencrypt to ZeroSSL Free SSL Certificates for old android accessing HTTPS based service

For some elder android devices,  like the OPDS reader app running on, the connection with calibre-web server suddenly broke. 

Encountering error of :

java.security.cert.CertPathValidatorException: Trust anchor for certification path not found

Could be fixed by following instructions:  

https://github.com/acmesh-official/acme.sh/issues/3790

remember to change the email account in acme.sh.env file.

Background information:

LetsEncrypt certificates fails on Android phones running Android 7 or older

https://community.letsencrypt.org/t/letsencrypt-certificates-fails-on-android-phones-running-android-7-or-older/205686