Make sure project is tested with Crystal 1.21

Author: ellmetha

.github/workflows/qa.yml

@@ -12,7 +12,7 @@ jobs:
   qa:
     runs-on: ubuntu-latest
     container:
-      image: crystallang/crystal:1.20.0
+      image: crystallang/crystal:1.21.0
     steps:
       - uses: actions/checkout@v1
       - name: Install shards

.github/workflows/specs.yml

@@ -15,9 +15,9 @@ jobs:
       fail-fast: false
       matrix:
         crystal: [
-          '1.18.1',
           '1.19.0',
           '1.20.0',
+          '1.21.0',
           'nightly',
         ]
     services: