[flake8-comprehensions] Stabilize unnecessary-dict-comprehension-for-iterable (C420) (#16624)

## Summary

Stabilizes C420 for the 0.10 release.

## Test Plan

No open issues or PRs (except a general issue about [string
normalization](#16579)). The
last (and only) false-negative bug fix was over a month ago.

The tests for this rule were already not on the `preview_rules` test, so
I just changed the `RuleGroup`. The documentation looked okay to me.

Author: ntBre

crates/ruff_linter/src/codes.rs

@@ -386,7 +386,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Flake8Comprehensions, "17") => (RuleGroup::Stable, rules::flake8_comprehensions::rules::UnnecessaryMap),
         (Flake8Comprehensions, "18") => (RuleGroup::Stable, rules::flake8_comprehensions::rules::UnnecessaryLiteralWithinDictCall),
         (Flake8Comprehensions, "19") => (RuleGroup::Stable, rules::flake8_comprehensions::rules::UnnecessaryComprehensionInCall),
-        (Flake8Comprehensions, "20") => (RuleGroup::Preview, rules::flake8_comprehensions::rules::UnnecessaryDictComprehensionForIterable),
+        (Flake8Comprehensions, "20") => (RuleGroup::Stable, rules::flake8_comprehensions::rules::UnnecessaryDictComprehensionForIterable),
 
         // flake8-debugger
         (Flake8Debugger, "0") => (RuleGroup::Stable, rules::flake8_debugger::rules::Debugger),