Replace GetOsPageSize with minipal_getpagesize

[pavelsavara]

Replace all usages of GetOsPageSize() (defined in src/coreclr/utilcode/util.cpp) with minipal_getpagesize() (defined in src/native/minipal/wasm.h) and delete the former.

minipal_getpagesize() already caches the value and returns the correct page size on all platforms including WASM.

Context: #127328 (comment)

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