Updated GH actions to use strings for php versions

Looked like 8.0 was being converted to 8

.github/workflows/phpunit.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        php: [7.3, 7.4, 8.0]
+        php: ['7.3', '7.4', '8.0']
     steps:
     - uses: actions/checkout@v1
 

.github/workflows/test-migrations.yml

@@ -17,7 +17,7 @@ jobs:
     runs-on: ubuntu-20.04
     strategy:
       matrix:
-        php: [7.3, 7.4, 8.0]
+        php: ['7.3', '7.4', '8.0']
     steps:
       - uses: actions/checkout@v1