fix: deactivate the deprecated Gatsby Cache plugin

[verythorough]

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This plugin was deprecated last year, and a deprecation message was added to the plugin description (#415). However, by deactivating the plugin, we can remove it from the directory for new installs. This will not remove the plugin from sites where it's already installed, and the plugin listing will still show in the list of installed plugins in the site admin UI.

Let me know if there's some reason why we shouldn't deactivate this!

[verythorough]

Adding @ascorbic for review since you made the deprecation PR and seem most likely to know if there's a reason not to deactivate.