Strengthen generic data types to a SHOULD

This was added as a MAY in 3.1.1 but we can make it a SHOULD in 3.2.

Author: handrews

src/oas.md

@@ -2632,7 +2632,7 @@ There are two ways to define the value of a discriminating property for an inher
 
 ###### Generic (Template) Data Structures
 
-Implementations MAY support defining generic or template data structures using JSON Schema's dynamic referencing feature:
+Implementations SHOULD support defining generic or template data structures using JSON Schema's dynamic referencing feature:
 
 * `$dynamicAnchor` identifies a set of possible schemas (including a default placeholder schema) to which a `$dynamicRef` can resolve
 * `$dynamicRef` resolves to the first matching `$dynamicAnchor` encountered on its path from the schema entry point to the reference, as described in the JSON Schema specification