[rust] Migrate to syn v2.

Strictly speaking, this isn't needed to fix gtest initializers, but it
will probably be helpful as that seems to be the general direction the
Rust ecosystem is moving.

Bug: 1466674
Change-Id: I02f4394067d5dbc3b0cf624fd8b9733ae1942eb3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4940774
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Łukasz Anforowicz <lukasza@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1210599}

testing/rust_gtest_interop/BUILD.gn

@@ -56,7 +56,7 @@ if (enable_rust) {
     deps = [
       "//third_party/rust/proc_macro2/v1:lib",
       "//third_party/rust/quote/v1:lib",
-      "//third_party/rust/syn/v1:lib",
+      "//third_party/rust/syn/v2:lib",
     ]
 
     # This target's contents are exposed as part of :rust_gtest_interop.