CI: bump crystal version in test matrix (#3769)

.github/workflows/ci.yml

@@ -38,10 +38,11 @@ jobs:
       matrix:
         stable: [true]
         crystal:
-          - 1.3.2
           - 1.4.1
           - 1.5.1
           - 1.6.2
+          - 1.7.3
+          - 1.8.1
         include:
           - crystal: nightly
             stable: false