fix: WWW-6 update newsletter subscription url

[r41ph]

Description

https://regennetwork.atlassian.net/browse/WWW-6

Author Checklist

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please add links to any relevant follow up issues.

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• provided a link to the relevant issue or specification
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• reviewed "Files changed" and left comments if necessary
• confirmed all CI checks have passed

How to test

1. Go to https://deploy-preview-2502--regen-website.netlify.app/ and at the bottom of the page chack that the subscription form works as expected.

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• confirmed all author checklist items have been addressed
• reviewed code correctness and readability
• verified React components follow DRY principles
• reviewed documentation is accurate
• reviewed tests
• manually tested (if applicable)

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[r41ph]

@erikalogie see testing instructions

[blushi]

utACK

[erikalogie]

LGTM!