Fix PHPUnit vulnerability: unsafe deserialization in PHPT test runner

Security alert for CVE affecting PHPUnit cleanupForCoverage() method.
Vulnerable versions: <= 8.5.51, <= 9.6.32, <= 10.5.61, <= 11.5.49, <= 12.5.7

Required dropping PHP 7.1 support (EOL since Dec 2019) to use patched PHPUnit versions.

Changes:
- Minimum PHP version: 7.2+ (was 7.1+)
- PHPUnit: 8.5.52+ or 9.6.33+ (was 7.5, 8.5.x, or 9.6.x)
- Removed PHP 7.1 from CI matrix

Author: erusev

.github/workflows/unit-tests.yaml

@@ -9,7 +9,6 @@ jobs:
         strategy:
             matrix:
                 php:
-                    - '7.1'
                     - '7.2'
                     - '7.3'
                     - '7.4'

composer.json

@@ -13,14 +13,14 @@
         }
     ],
     "require": {
-        "php": ">=7.1",
+        "php": ">=7.2",
         "ext-mbstring": "*"
     },
     "require-dev": {
-        "phpunit/phpunit": "^7.5|^8.5|^9.6"
+        "phpunit/phpunit": "^8.5.52|^9.6.33"
     },
     "autoload": {
-        "psr-0": {"Parsedown": ""}
+        "psr-0": { "Parsedown": "" }
     },
     "autoload-dev": {
         "psr-0": {