DEV: Specify the latest openapi spec version (PR #14012)

The latest openapi spec version is v3.1.0

OpenAPI Specification v3.1.0 | Introduction, Definitions, & More

Specifying the latest version will allow our openapi spec linter to use this version and allow use to use the new type format that allows for specifying a type as “null”, which we need because sometimes our api responses include null values instead of a “string”, “integer”, or “object” type.

See: How to define a property that can be string or null in OpenAPI (Swagger)? - Stack Overflow

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