docs(how-to/middleware): remove "(unstable)"

[brookslybrand]

It's redundant since we have the callout at the top and the 🧪 in the sidebar

[changeset-bot]

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[brookslybrand]

Just passing this by you first @brophdawg11 in case for some reason you felt middleware was due an exception

[brophdawg11]

oh yeah just an oversight, I wasn't sure if we were putting it in the heading of the page as well - but the warning below should be plenty