deps: Update com.google.auth dependencies to v1.20.0

[renovate-bot]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
com.google.auth:google-auth-library-oauth2-http	1.19.0 -> 1.20.0
com.google.auth:google-auth-library-credentials	1.19.0 -> 1.20.0

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Release Notes

googleapis/google-auth-library-java (com.google.auth:google-auth-library-oauth2-http)

v1.20.0

Features

• Byoid metrics framework (#​1232) (38bdf60)

Bug Fixes

• Make derived classes of CredentialSource public (#​1236) (9bb9e0a)

Documentation

• Update library definitions in README to the latest version (#​1239) (0c5cff2)

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