chore: update frontend environment variables

[oana-lolea]

Changes proposed in this pull request

• Updated frontend environment variables: GRAPHQL_URL, OPEN_PAYMENTS_URL, ENABLE_INSECURE_MESSAGE_COOKIE, NODE_ENV, KRATOS_ADMIN_URL.
• Changed start command in package.json.
• Updated docs with the new changes.

Context

For this issue, I made the following changes:

• Made GRAPHQL_URL and OPEN_PAYMENTS_URL mandatory and removed their default values.
• Set NODE_ENV to production in docker files.
• Set default value for ENABLE_INSECURE_MESSAGE_COOKIE to true in docker files and removed it from start script in package.json
• Added KRATOS_ADMIN_URL to variables in envConfig.server.ts.
• Updated frontend-variables.mdx.

Fixes #2994

Checklist

• [ ] Related issues linked using fixes #number
• [ ] Tests added/updated
• [ ] Make sure that all checks pass
• [ ] Bruno collection updated (if necessary)
• [ ] Documentation issue created with user-docs label (if necessary)
• [ ] OpenAPI specs updated (if necessary)

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[melissahenderson]

Related to #2994

[melissahenderson]

Doc updates are ready to go in PR #3042, but I'm waiting for this issue and #2994 to be merged first. (Please let me merge my PR when it's time, there are prettier issues I need to fix first).

[oana-lolea]

@melissahenderson merged!