Because of a mistake introduced into the SAM schema, the `AlexaSkill`
event type doesn't have any required properties anymore.

When the `CfnFunction` is trying all the different event types in the
type union that it supports, it will go through every type in
alphabetical order and pick the first type that doesn't fail its
validation.

After the schema change, the first type (`Alexa` which starts with an
`A`) would therefore accept all types: no required fields, and for
JavaScript compatibility purposes we allow superfluous fields, and so
we pick a type that doesn't render anything.

This change reorders the alternatives in the union such that stronger
types are tried first.

`HttpApiEvent` and `AlexaSkillEvent` both have no required
properties, and this now reverses the problem: `AlexaSkillEvent` can
no longer be specified because `HttpApiEvent` will match first.

But that's the more common use case, so better for now, while we wait
for the spec fix to come in, to prefer the HTTP API.

Relates to #26637.