composer.json: bump PHP requirement to 8.1

We have been using PHP 8.1 since #1176 (and implicitly requiring it
since #1184), but I forgot to bump the Composer requirement.

I discovered this coincidentally when Dependabot was failing to update
`overtrue/phplint` beyond v4.1.0. While investigating, I remembered
seeing that their v5.0.0 release had a PHP 8.1 requirement, which made
Dependabot think it was incompatible with our project due to the
(incorrect) 8.0+ requirement.

composer.json

@@ -10,7 +10,7 @@
 		"google/recaptcha": "1.2.4",
 		"league/oauth2-facebook": "2.2.0",
 		"league/oauth2-google": "4.0.0",
-		"php": "^8.0",
+		"php": "^8.1",
 		"php-di/php-di": "6.4.0",
 		"phpmailer/phpmailer": "6.6.0",
 		"team-reflex/discord-php": "7.0.9",