Update ekino/phpstan-banned-code requirement from ^1.0 to ^2.1 (#30)

Updates the requirements on [ekino/phpstan-banned-code](https://github.com/ekino/phpstan-banned-code) to permit the latest version.
- [Release notes](https://github.com/ekino/phpstan-banned-code/releases)
- [Changelog](https://github.com/ekino/phpstan-banned-code/blob/master/CHANGELOG.md)
- [Commits](ekino/phpstan-banned-code@v1.0.0...v2.1.0)

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

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

composer.json

@@ -50,7 +50,7 @@
     "require-dev": {
         "bnf/phpstan-psr-container": "^1.0",
         "doctrine/doctrine-bundle": "^2.10",
-        "ekino/phpstan-banned-code": "^1.0",
+        "ekino/phpstan-banned-code": "^2.1",
         "league/flysystem-memory": "^3.16",
         "mockery/mockery": "^1.6",
         "phpstan/phpstan": "^1.10.50",