Remove Stmt::TryStar (#6566)

## Summary

Instead, we set an `is_star` flag on `Stmt::Try`. This is similar to the
pattern we've migrated towards for `Stmt::For` (removing
`Stmt::AsyncFor`) and friends. While these are significant differences
for an interpreter, we tend to handle these cases identically or nearly
identically.

## Test Plan

`cargo test`

crates/ruff_python_semantic/src/analyze/branch_detection.rs

@@ -59,12 +59,6 @@ fn alternatives(stmt: &Stmt) -> Vec<Vec<&Stmt>> {
             handlers,
             orelse,
             ..
-        })
-        | Stmt::TryStar(ast::StmtTryStar {
-            body,
-            handlers,
-            orelse,
-            ..
         }) => vec![body.iter().chain(orelse.iter()).collect()]
             .into_iter()
             .chain(handlers.iter().map(|handler| {