Add age-based cleanup for simplecache

[be-student]

Fixes #2105.

SimpleCacheFileSystem does not persist cache metadata, so its inherited clear_expired_cache() had no records to inspect and silently retained every cached file. This override uses each cached file's local modification time, requires an explicit non-negative maximum age, and only modifies the writable cache location.

The regression tests cover files retained after writes, selective age-based removal, and invalid expiry arguments. The feature documentation and changelog now describe manual age-based cleanup.

Validation:

• pytest -q fsspec/implementations/tests/test_cached.py (79 passed, 31 skipped)
• all pre-commit hooks on changed files (including Ruff and codespell)
• full suite: 1562 passed, 163 skipped, 2 xfailed; the 52 failures are the existing macOS failures in test_posix_tests_bash_stat, reproduced unchanged on a clean upstream/master worktree because BSD stat does not support -c

[martindurant]

Runs into the following issue on windows (seen in the test):

>       os.utime(old_cached, (1, 1))
E       TypeError: utime: path should be string, bytes or os.PathLike, not NoneType