Update guzzlehttp/psr7 requirement from ^2.11 to ^2.12 (#70)

Updates the requirements on [guzzlehttp/psr7](https://github.com/guzzle/psr7) to permit the latest version.
- [Release notes](https://github.com/guzzle/psr7/releases)
- [Changelog](https://github.com/guzzle/psr7/blob/2.12/CHANGELOG.md)
- [Commits](guzzle/psr7@2.11.0...2.12.3)

---
updated-dependencies:
- dependency-name: guzzlehttp/psr7
  dependency-version: 2.12.3
  dependency-type: direct:development
...

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Author: dependabot[bot]

composer.json

@@ -9,7 +9,7 @@
 	"require-dev": {
 		"phpunit/phpunit": "^10.5",
 		"php-mock/php-mock-phpunit": "^2.15",
-		"guzzlehttp/psr7": "^2.11",
+		"guzzlehttp/psr7": "^2.12",
 		"corpus/coding-standard": "^0.10.0",
 		"squizlabs/php_codesniffer": "^4.0",
 		"friendsofphp/php-cs-fixer": "^3.95",