Deprecate InlineModelResolver and use flatten option provided by parser

[frantuma]

swagger-api/swagger-parser#1494 introduces flatten option (and related InlineModelResolver migrated from swagger-codegen) in swagger-parser v1; swagger-codegen 2.x and 3.x must be updated to:

1. Deprecate InlineModelResolver and clearly state that further development and maintenance will be targeting swagger-parser

2. Replace all occurrences of InlineModelResolver usage with an updated parser invocation with flatten = true, in both v2 and v3

[gracekarina]

closed by #10841