Update env files with google tag manager keys

[jtmst]

Requirements

ENV files need updated with google tag manager credentials

The following fields need updated in the corresponding env files for dev/staging/prod:
GOOGLE_TAG_MANAGER_ID=
GOOGLE_TAG_MANAGER_AUTH=
GOOGLE_TAG_MANAGER_PREVIEW=

These could all be stored in env files; the Analytics team has confirmed that they are not sensitive information and can exist within a code repo.

For example, this is how the key surfaces in Content Build currently: https://github.com/department-of-veterans-affairs/content-build/blob/main/src/site/assets/js/google-analytics/vagovdev.js

Please label these keys in a comment clearly as safe to be in code so that if they are caught by a secrets scan that we can point to that.

Acceptance criteria

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1. env.dev updated
2. env.staging updated
3. env.prod updated

Background & implementation details

Implementation tasks

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1. Use this if it helps you or feel free to delete.