fix(yaml-cfn): do not deserialize year-month-date as strings

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

packages/@aws-cdk/cloudformation-include/test/test-templates/yaml/year-month-date-as-strings.yaml

@@ -0,0 +1,14 @@
+AWSTemplateFormatVersion: 2010-09-09
+Resources:
+  Role:
+    Type: AWS::IAM::Role
+    Properties:
+      AssumeRolePolicyDocument:
+        Version: 2012-10-17
+        Statement:
+          - Effect: Allow
+            Principal:
+              Service:
+                - ec2.amazonaws.com
+            Action:
+              - sts:AssumeRole

packages/@aws-cdk/cloudformation-include/test/yaml-templates.test.ts

@@ -23,6 +23,33 @@ describe('CDK Include', () => {
     );
   });
 
+  test('can ingest a template with year-month-date parsed as string instead of Date', () => {
+    includeTestTemplate(stack, 'year-month-date-as-strings.yaml');
+
+    expect(stack).toMatchTemplate({
+      "AWSTemplateFormatVersion": "2010-09-09",
+      "Resources": {
+        "Role": {
+          "Type": "AWS::IAM::Role",
+          "Properties": {
+            "AssumeRolePolicyDocument": {
+              "Version": "2012-10-17",
+              "Statement": [
+                {
+                  "Effect": "Allow",
+                  "Principal": {
+                    "Service": ["ec2.amazonaws.com"],
+                  },
+                  "Action": ["sts:AssumeRole"],
+                },
+              ],
+            },
+          },
+        },
+      },
+    });
+  });
+
   test('can ingest a template with the short form Base64 function', () => {
     includeTestTemplate(stack, 'short-form-base64.yaml');
 

packages/@aws-cdk/yaml-cfn/lib/yaml.ts

@@ -54,6 +54,6 @@ const shortForms: yaml_types.Schema.CustomTag[] = [
 function parseYamlStrWithCfnTags(text: string): any {
   return yaml.parse(text, {
     customTags: shortForms,
-    schema: 'yaml-1.1',
+    schema: 'core',
   });
 }