Detect singular example in OpenAPI 3.1 schemas (ExampleSingularDeprecation rule)

[takayamaki]

ref: #152

Continuing the OpenAPI 3.1 work from #193.

This PR adds a new SpecValidator rule:

• OpenAPIParser::SpecValidator::Rules::ExampleSingularDeprecation

In 3.0, the singular example keyword on a Schema Object is the standard way to attach a sample value.
3.1 deprecates it in favor of the JSON Schema examples array.

The rule flags the presence of example on any schema in a 3.1 document.

# 3.1 document — flagged
schema:
  type: string
  example: ABC-123   # deprecated; use `examples: [ABC-123]

[geemus]

lgtm, thanks!