Deprecate usage-metric-dashboard extension

We want to deprecate the usage-metric-dashboard extension, although we have not fully landed on how exactly we will do it.

Easier/short-term option: Keep it entirely on the connect-extensions side — add a deprecation note to the README and update the item's description to point to the built-in feature and the Connect version that now includes it (a "carrot" to nudge the upgrade), optionally prefixing the title with [DEPRECATED]. This needs no product change and works on every Connect version.

Harder/long-term option: Product-side mechanism already filed here: https://github.com/posit-dev/connect/issues/40202 – a max_connect_version field so the Gallery automatically flags the item as superseded (pointing to the built-in alternative, and optionally hiding or badging it) once a server is past that version.

We also haven't decided on how long we'll let things stay deprecated before deletion. The leaning is to remove an item once the last Connect version that lacked the built-in feature is out of support, which lines up naturally with #40202's max_connect_version. FYI deletion is safe: anything a customer already deployed from the Gallery keeps working afterward (it's a static copy), and the source stays in the repo's git history. (It would be simpler to let deprecated items linger but stop updating them, but this would clutter the Gallery indefinitely.)

Start by reviewing the extension README and the item's description, then read the linked Connect issue #40202. The work is not ready to begin until the project chooses between the short-term documentation approach and the longer-term product-side mechanism, including a deletion timeline.