Stop claiming that dbt 1.8 is the latest version now that 1.9 has entered General Availability

[owenprough-sift]

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

• I have reviewed the Content style guide 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 and/or version a block 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."
• The content in this PR requires a dbt release note, so I added one to the release notes page.

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[runleonarun]

Thank you @owenprough-sift Great catch!

[runleonarun]

Looks good!