[TASK] Update phpstan/phpstan requirement from ^1.6.2 to ^1.7.15 (#464)

Updates the requirements on [phpstan/phpstan](https://github.com/phpstan/phpstan) to permit the latest version.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Changelog](https://github.com/phpstan/phpstan/blob/1.7.x/CHANGELOG.md)
- [Commits](https://github.com/phpstan/phpstan/compare/1.6.2...1.7.15)

---
updated-dependencies:
- dependency-name: phpstan/phpstan
  dependency-type: direct:development
...

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

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

composer.json

@@ -42,7 +42,7 @@
 		"jangregor/phpstan-prophecy": "^1.0.0",
 		"php-coveralls/php-coveralls": "^2.5.2",
 		"phpstan/extension-installer": "^1.1.0",
-		"phpstan/phpstan": "^1.6.2",
+		"phpstan/phpstan": "^1.7.15",
 		"phpstan/phpstan-phpunit": "^1.1.1",
 		"phpunit/phpunit": "^8.5.26",
 		"saschaegerer/phpstan-typo3": "^1.1.2",