[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/

crates/ruff_linter/src/codes.rs

@@ -552,7 +552,7 @@ pub fn code_to_rule(linter: Linter, code: &str) -> Option<(RuleGroup, Rule)> {
         (Pyupgrade, "045") => (RuleGroup::Preview, rules::pyupgrade::rules::NonPEP604AnnotationOptional),
         (Pyupgrade, "046") => (RuleGroup::Preview, rules::pyupgrade::rules::NonPEP695GenericClass),
         (Pyupgrade, "047") => (RuleGroup::Preview, rules::pyupgrade::rules::NonPEP695GenericFunction),
-        (Pyupgrade, "049") => (RuleGroup::Preview, rules::pyupgrade::rules::PrivateTypeParameter),
+        (Pyupgrade, "049") => (RuleGroup::Stable, rules::pyupgrade::rules::PrivateTypeParameter),
         (Pyupgrade, "050") => (RuleGroup::Preview, rules::pyupgrade::rules::UselessClassMetaclassType),
 
         // pydocstyle