chore(deps): update dependency gaxios to v6

[renovate]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
gaxios	^5.0.0 -> ^6.0.0

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Release Notes

googleapis/gaxios (gaxios)

v6.0.0

Compare Source

⚠ BREAKING CHANGES

• add status as a number to GaxiosError, change code to error code as a string (#​552)
• migrate to Node 14 (#​548)
• examine response content-type if no contentType is set (#​535)

Bug Fixes

• Add status as a number to GaxiosError, change code to error code as a string (#​552) (88ba2e9)
• Examine response content-type if no contentType is set (#​535) (cd8ca7b)

Miscellaneous Chores

• Migrate to Node 14 (#​548) (b9b26eb)

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