Drop PHP 7.2 from build PHP versions list

As per #165 we're no longer going to build this version. (Note that PHP 8 support is being worked on it #166.)

.github/workflows/ci.yml

@@ -43,7 +43,7 @@ jobs:
         name: Generate PHP
         shell: bash
         run: |
-          echo "::set-output name=versions::[\"7.4\", \"7.3\", \"7.2\"]"
+          echo "::set-output name=versions::[\"7.4\", \"7.3\"]"
   php-type-matrix:
     name: PHP Type Matrix
     runs-on: ubuntu-latest