ci: unmark solc 0.6 and 0.7 as experimental

Author: elopez

.github/workflows/ci.yml

@@ -157,10 +157,6 @@ jobs:
           - "0.6.12"
           - "0.7.5"
         include:
-          - solc: "0.6.12"
-            experimental: true
-          - solc: "0.7.5"
-            experimental: true
           - os: windows-latest
             experimental: true