Upgrade openapi-typescript dependency

[wilsonianb]

To include:

• Big honkin‘ refactor, OpenAPI 3.1 features openapi-ts/openapi-typescript#968

Currently not released yet: Released in v6.0.0+
https://www.npmjs.com/package/openapi-typescript

[wilsonianb]

Hopefully this isn't still an issue:

• feat(open-payments): adding open-payments package rafiki#669 (comment)

[wilsonianb]

I've started a branch for this:

• Bw oas ts wilsonianb/rafiki#162

but I appear to be running into:

• External interface has incorrect properties defined for local files that contains other openapi openapi-ts/openapi-typescript#993 (comment)

[wilsonianb]

• chore(deps): update dependency openapi-typescript to v6 rafiki#996

[wilsonianb]

• upgrade
• take advantage of the OpenAPI 3.1 benefits of Big honkin‘ refactor, OpenAPI 3.1 features openapi-ts/openapi-typescript#968
• determine if we still need to work around External interface has incorrect properties defined for local files that contains other openapi openapi-ts/openapi-typescript#993

[mkurapov]

• Just tried, External interface has incorrect properties defined for local files that contains other openapi openapi-ts/openapi-typescript#993 seems to be an issue still (as of v6.2.0)