Rename JoinedStr to FString in the AST (#6379)

## Summary

Per the proposal in https://github.com/astral-sh/ruff/discussions/6183,
this PR renames the `JoinedStr` node to `FString`.

crates/ruff_python_ast/src/visitor.rs

@@ -476,7 +476,7 @@ pub fn walk_expr<'a, V: Visitor<'a> + ?Sized>(visitor: &mut V, expr: &'a Expr) {
                 visitor.visit_format_spec(expr);
             }
         }
-        Expr::JoinedStr(ast::ExprJoinedStr { values, range: _ }) => {
+        Expr::FString(ast::ExprFString { values, range: _ }) => {
             for expr in values {
                 visitor.visit_expr(expr);
             }