fix(go): parsing of uuid in model AreaId in IaaS fails

[marceljk]

Testing instructions

1. Generate the Go SDK
2. Add in IaaS v1api, v2beta1api and v2api the following unit test:

func TestAreaId_UnmarshalJSON(t *testing.T) {
	type args struct {
		src []byte
	}
	tests := []struct {
		name    string
		args    args
		wantErr bool
	}{

		{
			name: "success - string d61a8564-c8dd-4ffb-bc15-143e7d0c85ed",
			args: args{
				src: []byte(`"d61a8564-c8dd-4ffb-bc15-143e7d0c85ed"`),
			},
			wantErr: false,
		},
		{
			name: "success - StaticAreaID PUBLIC",
			args: args{
				src: []byte(`"PUBLIC"`),
			},
			wantErr: false,
		},
		{
			name: "success - StaticAreaID SCHWARZ",
			args: args{
				src: []byte(`"SCHWARZ"`),
			},
			wantErr: false,
		},
	}
	for _, tt := range tests {
		t.Run(tt.name, func(t *testing.T) {
			v := &AreaId{}
			if err := v.UnmarshalJSON(tt.args.src); (err != nil) != tt.wantErr {
				t.Errorf("UnmarshalJSON() error = %v, wantErr %v", err, tt.wantErr)
			}
			marshalJson, err := v.MarshalJSON()
			if err != nil {
				t.Fatalf("failed marshalling AreaId: %v", err)
			}
			if string(marshalJson) != string(tt.args.src) {
				t.Fatalf("wanted %s, get %s", tt.args.src, marshalJson)
			}
		})
	}
}

3. Run the unit tests
   -> Should be successful
4. OPTIONAL: Checkout the branch oas-bot-31097659889/iaas from the Go SDK (Generator: Update SDK /services/iaas stackit-sdk-go#9754) and do again the steps 2 and 3. This time it will fail