Need to add Windows Sever 2025 for WebAssembly testing to images built for .NET

[artl93]

We need a new image added for Windows Server 2025 added to dotnet-buildtools-prereqs-docker.

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[ericstj]

Identified in #111938 (comment)

Seems to require a new image built for web assembly testing from https://github.com/dotnet/dotnet-buildtools-prereqs-docker/tree/f3a1d10babe6404a98a59d957f0c816de672d494/src/windowsservercore