[GO] fix post process file type

[mathewab]

fixes https://github.com/OpenAPITools/openapi-generator/issues/10718

The file type passed to postProcessFile by DefaultCodegen used to be supporting-mustache and supporting-common earlier. This has been changed to supporting-file in https://github.com/OpenAPITools/openapi-generator/pull/6357 and https://github.com/OpenAPITools/openapi-generator/pull/7366.

AbstractGoCodegen was never updated to match this change, and accepts only supporting-mustache. This PR changes it to accept supporting-file instead.

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[mathewab]

@antihax @grokify @kemokemo @jirikuncar @ph4r5h4d @lwj5

[wing328]

thanks for the fix, which looks good to me.