Replace Poco with std::filesystem in dynamic library loading code

[jclarkeSTFC]

Use std::filesystem methods and classes instead of Poco in the code loading dynamic libraries. Also threw in replacing boost string methods with std equivalents in DllOpen.cpp.

This is part of the work investigating #38011

To test:

Check libraries are loaded correctly by setting logging to Debug and checking that e.g. MantidCrystal.dll is loaded, along with all other .dll files in the bin folder. In a package some of those files will appear in the plugins folder.

This does not require release notes because it has no effect on the user.

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