[csharp] Fix enum default value

[jimschubert]

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

Based on the fix suggested in #4670 by @marekr.

I modified the test yaml locally by adding the following as a required property to Enum_Test in petstore-with-fake-endpoints-models-for-testing.yaml:

      just_symbol:
        type: string
        default: ">="
        enum:
          - ">="
          - "$"

This generated the expected greater than or equal enum value in the constructor's defaulted parameter list.

I've regenerated the client sample without this fix, because I didn't want to add the symbol-only enum with a default value and potentially break other tests (9 model tests consume this file).