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

test/lint/README.md

@@ -13,6 +13,14 @@ DOCKER_BUILDKIT=1 docker build -t bitcoin-linter --file "./ci/lint_imagefile" ./
 Building the container can be done every time, because it is fast when the
 result is cached and it prevents issues when the image changes.
 
+test runner
+===========
+
+To run the checks in the test runner outside the docker, use:
+
+```sh
+( cd ./test/lint/test_runner/ && cargo fmt && cargo clippy && cargo run )
+```
 
 check-doc.py
 ============