Require patched PHPUnit versions to fix CVE (unsafe deserialization in PHPT test runner)

Updates PHPUnit constraints to require versions with the fix for the unsafe deserialization vulnerability in the cleanupForCoverage() method:
- PHPUnit 8.5.52+ (was 8.5.x)
- PHPUnit 9.6.33+ (was 9.6.x)

Removes PHPUnit 7.x support as it is EOL and patch status unclear.

Vulnerability affects: <= 8.5.51, <= 9.6.32, <= 10.5.61, <= 11.5.49, <= 12.5.7

Author: erusev

composer.json

@@ -17,7 +17,7 @@
         "ext-mbstring": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": "^7.5|^8.5|^9.6"
+        "phpunit/phpunit": "^8.5.52|^9.6.33"
     },
     "autoload": {
         "psr-0": {"Parsedown": ""}