chore: update Nodejs auth library README to point to newer auth instructions

[piaxc]

Remove mention of service account key download and point to correct place for ADC setup.

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Fixes #<issue_number_goes_here> 🦕

[conventional-commit-lint-gcf]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot
https://conventionalcommits.org/

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• README.md - README.md is managed by synthtool. However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/main/.readme-partials.yaml

[danielbankhead]

Actually, could you update .readme-partials.yaml instead?