Certificate Authorities change which root CA certificates they use from time to time, so we have always provided a list of certificates that we currently use or may use in the future. Anybody using this list wont have to change anything. Google Trust Services offers certificates to Alphabet products and services including many Google Cloud services. We will begin rolling out certificate chains that use this cross-certificate in March 2021. We will slowly roll these changes out throughout the rest of the year and will complete them before December 15, 2021.”]
Source: https://security.googleblog.com/2021/03/google-https-and-device-compatibility.html