[csharp][generichost] fix syslib1045, use GeneratedRegex for .net7+

[ffabss]

Changed the ClientUtils of the generichost to use GeneratedRegex for the JsonRegex that checks if the content-type is JSON-like.
Since .NET 7 Source Generation can be used for Regex and now will be used for at least the JsonRegex of ClientUtils.
I also changed the JsonRegex from public to private because it is only used inside the ClientUtils.

The method SanitizeFilename also used a Regex but because it is unused I removed it instead of refactoring it.

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