Require phpspec/prophecy as a dev dependency

While e03ad06 (reverted) is not applied, we require phpspec/prophecy as
a development dependency. This is needed because phpunit dropped
ptophecy as a dependency in recent releases. See e03ad06 for further
reference.

composer.json

@@ -16,6 +16,7 @@
     "require-dev": {
         "doctrine/collections": "^1.6.8",
         "doctrine/common": "^2.13.3 || ^3.2.2",
+        "phpspec/prophecy": "^1.15",
         "phpunit/phpunit": "^7.5.20 || ^8.5.23 || ^9.5.13"
     },
     "conflict": {