Update auth URIs that have new oauth2.googleapis.com equivalents

[houglum]

As part of https://issuetracker.google.com/issues/79946689, we're updating several libraries to use the newer oauth2.googleapis.com-based OAuth2 URIs, moving away from the current accounts.google.com and googleapis.com/oauth2 equivalents.

Browser-interactive endpoints (like the auth endpoint, https://accounts.google.com/o/oauth2/auth) can remain unchanged. Additionally, some URIs don't have a oauth2.googleapis.com equivalent (notably, https://www.googleapis.com/oauth2/v<NUMBER>/certs and https://www.googleapis.com/oauth2/v<NUMBER>/userinfo, where <NUMBER> is... a number).

Occurrences can be found by greping for "accounts.google.com" or "googleapis.com/oauth2" within the source files.