ModelResolver.resolveExample() returns JsonNode but DateTimeSchema.cast() expects a String

I have this property in my POJO:

    @Schema(description = "Start of the shift as timestamp using ISO-8601 representation.",
        example = "\"2023-02-17T08:30:00Z\"")
    private Instant start;

When io.swagger.v3.core.jackson.ModelResolver.resolveExample() parses this, it returns a JsonNode (actually, TextNode):

                    ObjectMapper mapper = ObjectMapperFactory.buildStrictGenericObjectMapper();
                    return mapper.readTree(schema.example());
```

https://github.com/swagger-api/swagger-core/blob/c1950f506fedc4f7aa697fc88338494fd0d5526b/modules/swagger-core/src/main/java/io/swagger/v3/core/jackson/ModelResolver.java#L1709

This is then passed to `DateTimeSchema.cast()` which expects a String or Date or OffsetDateTime
https://github.com/swagger-api/swagger-core/blob/master/modules/swagger-models/src/main/java/io/swagger/v3/oas/models/media/DateTimeSchema.java#L37

Suggestions for a fix:

- Change the code to try specific Date types. If that doesn't work, use value.toString() and try to parse that
- Add JsonNode to the type switch.

Start with ModelResolver.resolveExample() in modules/swagger-core/src/main/java/io/swagger/v3/core/jackson/ModelResolver.java and DateTimeSchema.cast() in modules/swagger-models/src/main/java/io/swagger/v3/oas/models/media/DateTimeSchema.java. Reproduce the reported Instant example and trace the JsonNode/TextNode value between them. Done means the example is handled consistently for the reported date-time case without breaking existing accepted value types.