Update the warning about suspension of PR reviews from first-time contributors

[stevepiercy]

This message is obsolete and needs revision. It should align with the timeline set in https://6.docs.plone.org/volto/contributing/index.html.

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Caution

The Volto Team has suspended its review of new pull requests from first-time contributors until the release of Plone 7, which is preliminarily scheduled for the second quarter of 2026.
Read details.

Thanks for submitting your first pull request! You are awesome! 🤗

If you haven't done so already, read
Plone's Code of Conduct,
Contributing to Plone,
First-time contributors, and
Contributing to Volto,
as this will greatly help the review process.

Welcome to the Plone community! 🎉

Originally posted by @boring-cyborg[bot] in #8413 (comment)