Update multer requirement from 2.0 to 3.1

Updates the requirements on [multer](https://github.com/rwf2/multer) to permit the latest version.
- [Release notes](https://github.com/rwf2/multer/releases)
- [Commits](rwf2/multer@v2.0.0...v3.1.0)

---
updated-dependencies:
- dependency-name: multer
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

Cargo.toml

@@ -25,7 +25,7 @@ gotham = { version = "0.7.2", default-features = false }
 gotham_formdata_derive = { version = "0.0.7", path = "derive/" }
 log = "0.4"
 mime = "0.3.16"
-multer = "2.0"
+multer = "3.1"
 paste = "1.0"
 serde = "1.0"
 thiserror = "1.0"