Remove `field` preview include and text

[BillWagner]

When C# 14 is released, the include file /docs/csharp/includes/field-preview.md is obsolete. Remove it, and remove all references to it.

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Associated WorkItem - 505753

[BillWagner]

Now that C# 14 has been released, I changed references to field being available in C# 13 as a preview feature to "C# 14"