Fix ClassCastException in OpenAPINormalizer on composed schemas

[bjansen]

A ClassCastException can happen on OpenAPI 3.1 schemas when they declare both properties and oneOf, for example:

components:
  schemas:
    Payment:
      type: object
      properties:
        label:
          type: string
        otherLabel:
          type: string
        amount:
          type: number
      oneOf:
        - required:
            - label
            - amount
        - required:
            - otherLabel
            - amount

This raises the following exception:

java.lang.ClassCastException: class io.swagger.v3.oas.models.media.JsonSchema cannot be cast to class io.swagger.v3.oas.models.media.ComposedSchema (io.swagger.v3.oas.models.media.JsonSchema and io.swagger.v3.oas.models.media.ComposedSchema are in unnamed module of loader 'app')

	at org.openapitools.codegen.OpenAPINormalizer.normalizeSchema(OpenAPINormalizer.java:461)
	at org.openapitools.codegen.OpenAPINormalizer.normalizeComponentsSchemas(OpenAPINormalizer.java:419)
	at org.openapitools.codegen.OpenAPINormalizer.normalize(OpenAPINormalizer.java:222)

This cast is not necessary, and should even be avoided because in 3.1 ComposeSchema is not used as suggested in a comment in org.openapitools.codegen.utils.ModelUtils#isComposedSchema:

    // in 3.0, ComposeSchema is used for anyOf/oneOf/allOf
    // in 3.1, it's not the case so we need more checks below

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[wing328]

thanks for the fix, lgtm.