Update phpstan/phpstan requirement from ^0.12.25 to ^0.12.26

Updates the requirements on [phpstan/phpstan](https://github.com/phpstan/phpstan) to permit the latest version.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Commits](phpstan/phpstan@0.12.25...0.12.26)

Signed-off-by: dependabot-preview[bot] <support@dependabot.com>

composer.json

@@ -32,7 +32,7 @@
     "require-dev": {
         "jangregor/phpstan-prophecy": "^0.8.0",
         "phpstan/extension-installer": "^1.0.4",
-        "phpstan/phpstan": "^0.12.25",
+        "phpstan/phpstan": "^0.12.26",
         "phpunit/phpunit": "^8.5"
     },
     "config": {