Remove the deno adapter

[matthewp]

Changes

• The Deno team has taken over maintenance of the adapter, which is now here: https://github.com/denoland/deno-astro-adapter
• Needs Fix integration script for non-core adapters docs#4741

Testing

N/A

Docs

N/A

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Latest commit: 2008ae4

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[ematipico]

A reminder that when we publish a new version (major presumably), we have to update the index file to throw an error with a message saying that the package is deprecated, and moved to the new org.

[sarah11918]

Just a suggestion below, and pointing people to our own docs page if they need to remove/migrated.