Switch stabilization tests to new info

This commit switches the stabilization tests to the new stabilization
info file, meaning there's exactly one central location that decides if
things are stable. It also cleans the actual stabilization test code,
and documents what invariants we're trying to maintain.

extension/src/stabilization_info.rs

@@ -158,6 +158,7 @@ crate::functions_stabilized_at! {
         variance_x(statssummary2d,text),
         variance_y(statssummary2d,text),
         x_intercept(statssummary2d),
+        distinct_count(hyperloglog),
     }
 }