[csharp] Mustache fully qualify namespace for Path

[alfredo-accuris]

The generated code of Csharp get a fully quallify namespace for PATH

using Path = global::System.IO.Path;

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[alfredo-accuris]

Intends to fix #19344

[wing328]

thanks for the PR

with name mapping option for fallback, can this PR target upcoming minor release (breaking with fallback allowed)?

ref: https://github.com/openapitools/openapi-generator/blob/master/docs/customization.md#name-mapping

[devhl-labs]

This is not a breaking change. It is only fully qualifying a method, a very safe thing to do. I don't see any harm in merging it now.