bugfix: java @ApiResponse is now correctly using default responseCode and not falling back to 200

Created by: PirvuCatalin

According to https://spec.openapis.org/oas/v3.1.0#responses-object-example, it's possible to define both "200" and "default" possible responses. Unfortunately, the openapi-generator-cli for java code is incorrectly mapping "default" to "200" and not taking into account the fact that io.swagger.v3.oas.annotations.responses.ApiResponse also has a "default" value.

Currently, if you have an openapi file which defines cases "200" and "default", it will generate something as following:

 @Override
    @Operation(
            operationId = "getMethod1",
            tags = {"Method"},
            responses = {
                    @ApiResponse(responseCode = "200", description = "successful operation", content = {
                            @Content(mediaType = "application/json", schema = @Schema(implementation = Method1.class))
                    }),
                    @ApiResponse(responseCode = "200", description = "Error getting response", content = {
                            @Content(mediaType = "application/json", schema = @Schema(implementation = ErrorResponse.class))
                    })
            }
    )

, which obviously is incorrect (notice the duplicated responseCode). This PR is fixing that issue for java. There is a similar issue mentioned at https://github.com/OpenAPITools/openapi-generator/issues/12481 for python-fastapi and I might also fix that if I get a positive feedback on this PR.

TC mentions: @bbdouglas (2017/07) @sreeshas (2017/08) @jfiala (2017/08) @lukoyanov (2017/09) @cbornet (2017/09) @jeff9finger (2018/01) @karismann (2019/03) @Zomzog (2019/04) @lwlee2608 (2019/10)

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