Revert "fix: update _GOOGLE_OAUTH2_CERTS_URL (#365)"

[k-yone]

Issue

The new certificate (https://www.googleapis.com/oauth2/v3/certs) is in a jwk format and it doesn't match the format expected when decoding a jwt here.
https://github.com/googleapis/google-auth-library-python/blob/master/google/auth/jwt.py#L220

Fix

A quick fix is to revert this update and continue using the /v1 uri.
The decode() function mentioned above should be fixed to accept jwk format certificates in the future.

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[k-yone]

@googlebot I signed it!

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[busunkim96]

Thank you for the patch @k-yone!