deps: bump kin-openapi to v0.143.0 (#1105)

v0.143.0 removes the public openapi3.StringMap type (#1219), so
mappingToStringMap takes the plain map type. Body unchanged; the named
StringMap[MappingRef] was already assignable to map[string]MappingRef.

The release also brings T.WalkParameters (#1224), the arbitrary-key $ref
origin fix (#1227), and stable validation-error codes (#1223), consumed in
follow-up changes.