[csharp] Output DateOnly paramater in the correct default format (#19728)

[alec-petersen]

The DateOnly object was previously unhandled when being converted to a string in ParameterToString. This PR adds handling for DateOnly in the same way the other date objects are handled and outputs it in the correct default format. This conditional is only added if we are net60 or later.

For issue #19728

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[alec-petersen]

@devhl-labs Think I'm supposed to tag you on this one.

[devhl-labs]

I am not an approver but this is fine.

[alec-petersen]

@devhl-labs My bad, and thanks! @mandrean @shibayan @Blackclaws @lucamazzanti @iBicha , would it be possible to get a review on this?