Add 8.4 to matrix

Author: eiriksm

.github/workflows/tests.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        php: ['7.1', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2', '8.3']
+        php: ['7.1', '7.2', '7.3', '7.4', '8.0', '8.1', '8.2', '8.3', '8.4']
 
     steps:
       - name: Checkout code