chore(deps): bump syn from 1.0.109 to 2.0.18 in /backend (#1689)

Bumps [syn](https://github.com/dtolnay/syn) from 1.0.109 to 2.0.18.
- [Release notes](https://github.com/dtolnay/syn/releases)
- [Commits](https://github.com/dtolnay/syn/compare/1.0.109...2.0.18)

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

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

backend/Cargo.lock

@@ -5312,7 +5312,7 @@ dependencies = [
  "reqwest",
  "serde",
  "serde_json",
- "syn 1.0.109",
+ "syn 2.0.18",
  "uuid 1.3.4",
 ]
 

backend/windmill-api-client/Cargo.toml

@@ -24,4 +24,4 @@ base64.workspace = true
 prettyplease = "0.1.25"
 progenitor = { git = "https://github.com/oxidecomputer/progenitor", rev = "3d96016ae8d422e90513b2d34fb5b63eeab30b01" }
 serde_json = "1.0"
-syn = "1.0"
+syn = "2.0"