Update Slack channel links in roadmap.mdx

[borisno2]

update roadmap.mdx so KeystoneJS Slack has the correct links

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[JedWatson]

Thanks for catching that @borisno2

[JedWatson]

I made a minor edit, because I believe (@bladey to confirm) we're trying to point visitors at Slack over GH discussions.

Reason being all the channels are pretty fragmented at the moment so we're tying to consolidate on a few before expanding it again.