Use slither 0.8.3 in CI

Use the latest version of slither 0.8.3

Author: nkuba

Diff for: .github/workflows/contracts.yaml

@@ -228,7 +228,7 @@ jobs:
 
       - name: Install Slither
         env:
-          SLITHER_VERSION: 0.8.0
+          SLITHER_VERSION: 0.8.3
         run: pip3 install slither-analyzer==$SLITHER_VERSION
 
       - name: Install dependencies