[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
