If you want to use HTTP2 you don't need to configure anything in Prestashop. You just have to enable the mod_http2 in the Apache Web Server. You will fin more information about it at:
Please note that mod_http2 is experimental. Its behaviors, directives, and defaults are subject to more change from release to release relative to other standard modules.
You will find the main Apache configuration file at /opt/bitnami/apache2/conf/httpd.conf on your server. You can find more information about the Apache Web Server included in the Bitnami Google Cloud Image for Prestashop at:
I hope this information is useful for you. If you have any other question, please do not hesitate to let us know.