Currently, we are using the `yaml-1.1` schema when de-serializing YAML documents,
Unfortunately, this has the side effect of treating unquoted parts of the template like '2010-09-09'
as Date objects, instead of just simple strings.
This has been noted by a customer using the cloudformation-include module,
but I assume a very similar problem exists for other places we parse YAML,
like cloudformation-diff.

Switch to the `core` schema from `yaml-1.1`,
where those are treated as strings, instead of dates.

Fixes aws#13709