Bump phpstan/phpstan from 0.11.12 to 0.11.15

Bumps [phpstan/phpstan](https://github.com/phpstan/phpstan) from 0.11.12 to 0.11.15.
- [Release notes](https://github.com/phpstan/phpstan/releases)
- [Commits](phpstan/phpstan@0.11.12...0.11.15)

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