Stop claiming that dbt 1.8 is the latest version now that 1.9 has ent…

…ered General Availability (#6646)

`1.8` is no longer "latest". Long live `1.9`!

## What are you changing in this pull request and why?
I adjust the label of the 1.8 section of the upgrade guide to no longer
claim that 1.8 is the latest version of dbt.

I considered adding "(latest)" to the 1.9 page, but figured that we
could save 1 PR per dbt minor version by omitting it.

## Checklist
- [x] 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.
- [x] 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."
- [ ] The content in this PR requires a dbt release note, so I added one
to the [release notes
page](https://docs.getdbt.com/docs/dbt-versions/dbt-cloud-release-notes).
<!--
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website/docs/docs/dbt-versions/core-upgrade/07-upgrading-to-v1.8.md

@@ -1,5 +1,5 @@
 ---
-title: "Upgrading to v1.8 (latest)"
+title: "Upgrading to v1.8"
 id: upgrading-to-v1.8
 description: New features and changes in dbt Core v1.8
 displayed_sidebar: "docs"