fix(deps): update dependency google-auth-library to v6

[renovate-bot]

This PR contains the following updates:

Package	Type	Update	Change
google-auth-library	dependencies	major	^5.0.0 -> ^6.0.0

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Release Notes

googleapis/google-auth-library-nodejs

v6.0.0

Compare Source

⚠ BREAKING CHANGES

• typescript@3.7.x introduced some breaking changes in
  generated code.
• require node 10 in engines field (#​926)
• remove deprecated methods (#​906)

Features

• require node 10 in engines field (#​926) (d89c59a)

Bug Fixes

• do not warn for SDK creds (#​905) (9536840)
• use iamcredentials API to sign blobs (#​908) (7b8e4c5)
• deps: update dependency gaxios to v3 (#​917) (1f4bf61)
• deps: update dependency gcp-metadata to v4 (#​918) (d337131)
• types: add additional fields to TokenInfo (#​907) (5b48eb8)

Build System

• update to latest gts and TypeScript (#​927) (e11e18c)

Miscellaneous Chores

• remove deprecated methods (#​906) (f453fb7)

5.10.1 (2020-02-25)

Bug Fixes

• if GCF environment detected, increase library timeout (#​899) (2577ff2)

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