Support OpenAPI 3.1

OpenAPI 3.1 has just been released... Can we hope for support in this library?

Changes in the spec seems really small, but there are some small BCs that could complicate the stuff further: https://www.openapis.org/blog/2021/02/16/migrating-from-openapi-3-0-to-3-1-0

Main differences:
* no more `nullable`
```diff
-type: string
-nullable: true
+type:
+ - "string"
+ - "null"
```
* `exclusiveMinimum` and `exclusiveMaximum` are used directly
```diff
-minimum: 7
-exclusiveMinimum: true
+exclusiveMinimum: 7
```
* `examples`, no more `example`
```diff
-example: fedora
+examples:
+ - fedora
```
* Describing File Upload Payloads
* declaring JSON Schema dialect explicitly

Start by reviewing the OpenAPI 3.0-to-3.1 migration notes linked in the issue and comparing them with this library’s current OpenAPI handling. Done means the library supports the listed 3.1 changes, including nullable types, direct exclusive bounds, examples, file upload payloads, and explicit JSON Schema dialects.

索引モデルが issue の本文から書いたものです。