Merge bitcoin/bitcoin#28076: util: Replace std::filesystem with util/fs.h

bbbbdb0cd5 ci: Add filesystem lint check (MarcoFalke)
fada2f9110 refactor: Replace <filesystem> with <util/fs.h> (MarcoFalke)

Pull request description:

  Using `std::filesystem` is problematic:

  * There is a `fs` namespace wrapper for it. So having two ways to achieve the same is confusing.
  * Not using the `fs` wrapper is dangerous and buggy, because it disables known bugs by deleting problematic functions.

  Fix all issues by removing use of it and adding a linter to avoid using it again in the future.

ACKs for top commit:
  TheCharlatan:
    ACK  bbbbdb0cd5
  fanquake:
    ACK bbbbdb0cd5 🦀

Tree-SHA512: 0e2d49742b08eb2635e6fce41485277cb9c40fe20b81017c391d3472a43787db1278a236825714ca1e41c9d2f59913865cfb0c649e3c8ab1fb598c849f80c660

.cirrus.yml

@@ -84,6 +84,9 @@ task:
     memory: 1G
   # For faster CI feedback, immediately schedule the linters
   << : *CREDITS_TEMPLATE
+  test_runner_cache:
+    folder: "/lint_test_runner"
+    fingerprint_script: echo $CIRRUS_TASK_NAME $(git rev-parse HEAD:test/lint/test_runner)
   python_cache:
     folder: "/python_build"
     fingerprint_script: cat .python-version /etc/os-release