fix: Add prefix and suffix to discriminator

[MatthiasGabriel]

Use the existing toModelName method to normalize the discriminator

I stumbled upon the issue #16391 when I was trying to use an Enum as a discriminator which was added in #13846

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[MatthiasGabriel]

@welshm @cachescrubber @ddoming73 @wing328 any feedback to this mini MR?
Sorry to tag you directly, but you were involved in the original MR...

[wing328]

cc @OpenAPITools/generator-core-team

[wing328]

lgtm. let's give it a try.