Fix Rust GHA build by enabling "full" feature for syn. (#28717)

We must have been accidentally inheriting this feature being enabled from one of our deps, who have since turned off the "full" feature causing a break.

PiperOrigin-RevId: 950977436

Co-authored-by: Protobuf Team Bot <protobuf-github-bot@google.com>

MODULE.bazel

diff --git a/MODULE.bazel b/MODULE.bazel
index 7a6c7cf..d6eebf5 100644
--- a/MODULE.bazel
+++ b/MODULE.bazel
@@ -213,7 +213,7 @@
 )
 crate.spec(
     package = "syn",
-    version = ">=2",
+    version = "2",
 )
 crate.from_specs()
 use_repo(crate, crate_index = "crates")