Support Bean Validation for Maps' generic parameter are ignored

[mpressmar]

While validation annotations of generic parameters are now supported for Optionals and Array-like structures (see #4373) that is not the case for Map structures.

Example:

@NotNull private final Map<Integer, @NotNull BigDecimal> vintages;

will produce an OpenAPI specification:

"vintages":{"type":"object","additionalProperties":{"type":"number"}}

Any annotation on the Map's value is ignored.