Removing versions (#6021)

## What are you changing in this pull request and why?

Now that we have the option for dbt Cloud in the docs, we are removing
the v1.9 beta content until the beta is actually out.

This removes the whole upgrade guide since the sidebar is autogenerated.

## Checklist
- [ ] I have reviewed the [Content style
guide](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/content-style-guide.md)
so my content adheres to these guidelines.
- [ ] The topic I'm writing about is for specific dbt version(s) and I
have versioned it according to the [version a whole
page](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#adding-a-new-version)
and/or [version a block of
content](https://github.com/dbt-labs/docs.getdbt.com/blob/current/contributing/single-sourcing-content.md#versioning-blocks-of-content)
guidelines.
- [ ] I have added checklist item(s) to this list for anything anything
that needs to happen before this PR is merged, such as "needs technical
review" or "change base branch."
<!--
PRE-RELEASE VERSION OF dbt (if so, uncomment):
- [ ] Add a note to the prerelease version [Migration
Guide](https://github.com/dbt-labs/docs.getdbt.com/tree/current/website/docs/docs/dbt-versions/core-upgrade)
-->
<!-- 
ADDING OR REMOVING PAGES (if so, uncomment):
- [ ] Add/remove page in `website/sidebars.js`
- [ ] Provide a unique filename for new pages
- [ ] Add an entry for deleted pages in `website/vercel.json`
- [ ] Run link testing locally with `npm run build` to update the links
that point to deleted pages
-->

website/dbt-versions.js

@@ -16,10 +16,6 @@ exports.versions = [
     version: "1.9.1",
     customDisplay: "Cloud (Versionless)",
   },
-  {
-    version: "1.9",
-    isPrerelease: true,
-  },
   {
     version: "1.8",
     EOLDate: "2025-04-15",