[pyupgrade] Stabilize private-type-parameter (UP049) (#18515)
## Summary
Stabilizes the UP049 rule (private-type-parameter) by moving it from
Preview to Stable.
UP049 detects and fixes the use of private type parameters (those with
leading underscores) in PEP 695 generic classes and functions.
## Test plan
- Verified that UP049 tests pass:
`crates/ruff_linter/src/rules/pyupgrade/mod.rs`
- Ran full test suite with `make test`
- Confirmed that no test migration was needed as UP049 was already in
the main `rules` test function
## Rule documentation
https://docs.astral.sh/ruff/rules/private-type-parameter/
