[refurb] Stabilize `fromisoformat-replace-z` (FURB162) (#18510)

This PR stabilizes the FURB162 rule by moving it from preview to stable
status for the 0.12.0 release.

## Summary
- **Rule**: FURB162 (`fromisoformat-replace-z`)
- **Purpose**: Detects unnecessary timezone replacement operations when
calling `datetime.fromisoformat()`
- **Change**: Move from `RuleGroup::Preview` to `RuleGroup::Stable` in
`codes.rs`

## Verification Links
- **Tests**:
[refurb/mod.rs](https://github.com/astral-sh/ruff/blob/main/crates/ruff_linter/src/rules/refurb/mod.rs#L54)
- Confirms FURB162 has only standard tests, no preview-specific test
cases
- **Documentation**:
https://docs.astral.sh/ruff/rules/fromisoformat-replace-z/ - Current
documentation shows preview status that will be automatically updated

crates/ruff_linter/src/codes.rs

@@ -1130,7 +1130,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Refurb, "156") => (RuleGroup::Preview, rules::refurb::rules::HardcodedStringCharset),
         (Refurb, "157") => (RuleGroup::Preview, rules::refurb::rules::VerboseDecimalConstructor),
         (Refurb, "161") => (RuleGroup::Stable, rules::refurb::rules::BitCount),
-        (Refurb, "162") => (RuleGroup::Preview, rules::refurb::rules::FromisoformatReplaceZ),
+        (Refurb, "162") => (RuleGroup::Stable, rules::refurb::rules::FromisoformatReplaceZ),
         (Refurb, "163") => (RuleGroup::Stable, rules::refurb::rules::RedundantLogBase),
         (Refurb, "164") => (RuleGroup::Preview, rules::refurb::rules::UnnecessaryFromFloat),
         (Refurb, "166") => (RuleGroup::Preview, rules::refurb::rules::IntOnSlicedStr),