"required" property in Schema.java should be a Set (SortedSet for example), not a List

[diyan-gochev]

https://github.com/swagger-api/swagger-core/blob/b5188480f10a95eab0d26b845a4928a5258903a4/modules/swagger-models/src/main/java/io/swagger/v3/oas/models/media/Schema.java#L94C4-L94C42