enable testing on PHP 8.3 and 8.4

at this time both are still installed out of nightly builds, though 8.3
got released last week, see:
https://github.com/shivammathur/setup-php#tada-php-support

.github/workflows/tests.yml

@@ -19,7 +19,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        php-versions: ['7.3', '7.4', '8.0', '8.1', '8.2']
+        php-versions: ['7.3', '7.4', '8.0', '8.1', '8.2', '8.3', '8.4']
     name: PHP ${{ matrix.php-versions }} unit tests on ${{ matrix.operating-system }}
     env:
       extensions: gd, sqlite3