[ruff] Stabilize unnecessary-nested-literal (RUF041) (#16648)

Summary
--

Stabilizes RUF041. The tests are already in the right place, and the
docs look good.

Test Plan
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0 issues, 1 [PR] fixing nested literals and unions the day after the
rule was added. No changes since then

I wonder if the fix in that PR could be relevant for
#16639, where I noticed a
potential issue with `Union`. It could be unrelated, though.

[PR]: #14641

Author: ntBre

crates/ruff_linter/src/codes.rs

@@ -998,7 +998,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Ruff, "038") => (RuleGroup::Preview, rules::ruff::rules::RedundantBoolLiteral),
         (Ruff, "039") => (RuleGroup::Preview, rules::ruff::rules::UnrawRePattern),
         (Ruff, "040") => (RuleGroup::Stable, rules::ruff::rules::InvalidAssertMessageLiteralArgument),
-        (Ruff, "041") => (RuleGroup::Preview, rules::ruff::rules::UnnecessaryNestedLiteral),
+        (Ruff, "041") => (RuleGroup::Stable, rules::ruff::rules::UnnecessaryNestedLiteral),
         (Ruff, "043") => (RuleGroup::Preview, rules::ruff::rules::PytestRaisesAmbiguousPattern),
         (Ruff, "045") => (RuleGroup::Preview, rules::ruff::rules::ImplicitClassVarInDataclass),
         (Ruff, "046") => (RuleGroup::Stable, rules::ruff::rules::UnnecessaryCastToInt),