Remove "next" site

[jtcohen6]

Contributions

• I have read the contribution docs, and understand what's expected of me.

What page(s) or areas on docs.getdbt.com are affected?

Now that docs are versioned, we no longer need the twin deployments at docs.getdbt.com and next.docs.getdbt.com... right?

What changes are you suggesting?

We can remove logic put by yours truly in:

• docusaurus.config.js
• netlify.toml (PRERELEASE = "true")
• Netlify to deploy the next branch
• Algolia, which runs a separate search index for the "next" site... that never worked quite right

Let's also:

• update the PR template, to mention VersionBlock instead of the next branch
• set up a CNAME redirect from next.docs.getdbt.com to docs.getdbt.com
• just have one branch named main :)

Additional information

No response