Upgrade phpunit.

Need to turn off deprecation warnings for tests to pass (e.g. some
output in the middle of a string being tested on), but will need to be
reactivated and issues fixed before PHP9.

composer.json

@@ -5,7 +5,7 @@
         }
     },
     "require": {
-        "php": ">=5.4",
+        "php": ">=7.3",
         "filp/whoops": "2.*",
         "ircmaxell/password-compat": "1.0.4",
         "facebook/graph-sdk": "^5.6",
@@ -15,13 +15,12 @@
         "phpmailer/phpmailer": "^6.5"
     },
     "require-dev": {
-        "phpunit/phpunit": "4.8.*",
-        "phpunit/dbunit": ">=1.2",
-        "phpdocumentor/phpdocumentor": "2.*"
+        "phpunit/phpunit": "9.*",
+        "phpdocumentor/phpdocumentor": "3.*"
     },
     "config": {
         "platform": {
-            "php": "7.0"
+            "php": "7.3"
         }
     }
 }