Bump syn from 1.0.90 to 1.0.91 (#5273)

Bumps [syn](https://github.com/dtolnay/syn) from 1.0.90 to 1.0.91.
- [Release notes](https://github.com/dtolnay/syn/releases)
- [Commits](https://github.com/dtolnay/syn/compare/1.0.90...1.0.91)

---
updated-dependencies:
- dependency-name: syn
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

polkadot/Cargo.lock

@@ -11103,9 +11103,9 @@ dependencies = [
 
 [[package]]
 name = "syn"
-version = "1.0.90"
+version = "1.0.91"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "704df27628939572cd88d33f171cd6f896f4eaca85252c6e0a72d8d8287ee86f"
+checksum = "b683b2b825c8eef438b77c36a06dc262294da3d5a5813fac20da149241dcd44d"
 dependencies = [
  "proc-macro2",
  "quote",