Remove std::filesystem hacks

Lars Bilkerequested to merge
bilke/ogs:rm-fs into master

std::filesystem can now be used by all our minimum compilers (just had to bump gcc from 9.0 to 9.1). Removed logic for older compilers.

Fixes #3246 (closed).

  1. Feature description was added to the changelog
  2. Tests covering your feature were added?
Edited by Lars Bilke

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