ci: bump MSRV to 1.28

Author: cuviper

.github/workflows/ci.yml

@@ -17,7 +17,7 @@ jobs:
     strategy:
       matrix:
         include:
-          - rust: 1.6.0 # MSRV
+          - rust: 1.28.0 # MSRV
           - rust: stable
           - rust: beta
           - rust: nightly