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/preorder.rs

@@ -570,7 +570,7 @@ where
             }
         }
 
-        Expr::JoinedStr(ast::ExprJoinedStr { values, range: _ }) => {
+        Expr::FString(ast::ExprFString { values, range: _ }) => {
             for expr in values {
                 visitor.visit_expr(expr);
             }