workflows/ci.yml: Add multiple Go versions to testing matrix

This change officially removes Go 1.15 from the testing matrix and adds the Go versions used
for supporting the Exercism CLI. Namely Go 1.17, 1.18, and 1.19. Testing support for Go 1.20
and 1.21.x to follow.

.github/workflows/ci.yml

@@ -17,7 +17,10 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        go-version: ["1.15"]
+        go-version:
+          '1.17'
+          '1.18'
+          '1.19'
         os: [ubuntu-latest, windows-latest, macOS-latest]
 
     steps: