Pin the Composer platform to PHP 8.1

composer.json declares "php": ">=8.1", but resolution ran against whatever
PHP the machine doing the update happened to have. Running composer update
on 8.5 could therefore produce a lock containing a package that requires 8.2
or newer, which installs fine locally and fails on the 8.1 leg of the test
matrix — a lock that is only discovered to be wrong in CI.

config.platform.php makes resolution always target the declared minimum, so
a lock that resolves at all is a lock that installs everywhere the plugin
claims to run.

No package versions change: nothing currently locked requires a PHP newer
than 8.1, so this only records platform-overrides and the new content hash.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01D9acV1mHktGAb1uyvNmrR2

composer.json

@@ -36,6 +36,9 @@
         "build": "bash bin/build-zip.sh"
     },
     "config": {
+        "platform": {
+            "php": "8.1"
+        },
         "allow-plugins": {
             "dealerdirect/phpcodesniffer-composer-installer": true
         }

composer.lock

@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "847e27b3669444ab5ee66b2e07cd7b3e",
+    "content-hash": "24094d05e029ee5534769a89f1dbad12",
     "packages": [
         {
             "name": "stripe/stripe-php",
@@ -2593,5 +2593,8 @@
         "php": ">=8.1"
     },
     "platform-dev": {},
+    "platform-overrides": {
+        "php": "8.1"
+    },
     "plugin-api-version": "2.9.0"
 }