deps: update base64ct requirement from =1.8.1 to =1.8.3 in /backend

Updates the requirements on [base64ct](https://github.com/RustCrypto/formats) to permit the latest version.
- [Commits](https://github.com/RustCrypto/formats/commits/base64ct/v1.8.3)

---
updated-dependencies:
- dependency-name: base64ct
  dependency-version: 1.8.3
  dependency-type: direct:production
...

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

Author: dependabot[bot]

backend/Cargo.toml

@@ -51,7 +51,7 @@ rand = "0.8"
 version = "=0.5.12"
 
 [dependencies.base64ct]
-version = "=1.8.1"
+version = "=1.8.3"
 default-features = false
 features = ["alloc"]