The file C:\System32\pnidui.dll is not present in versions 24H2 and 25H2. It was used in Windows 7 and later up to Windows 11 23H2 and contained the network icons, not the pin icon. To replace the network icons you need a copy of pnidui.dll which is provided by third-party tools. One of them is Explorer Patcher and you can find it in C:\Program Files\ExplorerPatcher\pnidui.dll
I haven't checked, but the pin icon should be in file C:\Windows\SystemResources\imageres.dll.mun which CANNOT be modified directly, or you get an error and screws all your system icons! You must copy the file to another folder, modify it and then take ownership of both the specific file and the containing folder and grant full access to Administrators group for both the specific file and the containing folder. Even then, you cannot just copy-paste and replace it because it is used by File Explorer. So you must use a third party file manager or do it from Command Prompt while you have terminated File Explorer (End Task). Yes, it is a little complicated, but it works and it's worth it to replace all those flat Windows 11 default system icons with custom. See my older posts in Personalization section for details.