[flake8-use-pathlib] Stabilize os-listdir (PTH208) (#16642)

Summary
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Stabilizes PTH208. The test was already in the right place, and the docs
look good.

Test Plan
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0 issues and PRs, open or closed

Author: ntBre

crates/ruff_linter/src/codes.rs

@@ -922,7 +922,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Flake8UsePathlib, "205") => (RuleGroup::Stable, rules::flake8_use_pathlib::rules::OsPathGetctime),
         (Flake8UsePathlib, "206") => (RuleGroup::Stable, rules::flake8_use_pathlib::rules::OsSepSplit),
         (Flake8UsePathlib, "207") => (RuleGroup::Stable, rules::flake8_use_pathlib::rules::Glob),
-        (Flake8UsePathlib, "208") => (RuleGroup::Preview, rules::flake8_use_pathlib::violations::OsListdir),
+        (Flake8UsePathlib, "208") => (RuleGroup::Stable, rules::flake8_use_pathlib::violations::OsListdir),
         (Flake8UsePathlib, "210") => (RuleGroup::Preview, rules::flake8_use_pathlib::rules::InvalidPathlibWithSuffix),
 
         // flake8-logging-format