Update to syn 2

test_suite/tests/ui/unexpected-literal/container.stderr

@@ -1,4 +1,4 @@
-error: unexpected literal in serde container attribute
+error: expected path
  --> tests/ui/unexpected-literal/container.rs:4:9
   |
 4 | #[serde("literal")]

test_suite/tests/ui/unexpected-literal/field.stderr

@@ -1,4 +1,4 @@
-error: unexpected literal in serde field attribute
+error: expected path
  --> tests/ui/unexpected-literal/field.rs:5:13
   |
 5 |     #[serde("literal")]

test_suite/tests/ui/unexpected-literal/variant.stderr

@@ -1,4 +1,4 @@
-error: unexpected literal in serde variant attribute
+error: expected path
  --> tests/ui/unexpected-literal/variant.rs:5:13
   |
 5 |     #[serde("literal")]