Update stripe-php from 17.6.0 to 21.2.1

Four majors behind on the plugin's only production dependency. No site is
using Stripe yet and the integration has not been verified end to end, so
the pinned API version can move now at close to zero cost — later it would
mean coordinating with live payment traffic.

The library surface this plugin uses is four symbols, all in StripeGateway:
StripeClient, paymentIntents->create, Webhook::constructEvent and Event.
Checked each against the installed v21 by reflection; all present with
compatible signatures. Most of the breakage flagged across 18 to 21 is in
the V2 Events API, which this does not use — webhooks go through the V1
Webhook::constructEvent path.

The pinned Stripe API version moves to 2026-07-29.dahlia.

Verified beyond the suite, which mocks the client: a real HMAC-signed
payload round-trips through Webhook::constructEvent and returns a
Stripe\Event with the expected type and data, and a forged signature still
raises SignatureVerificationException. The 21.0.0 ErrorObject retyping to
null|string produces no new PHPStan findings.

Part of #194

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

CHANGELOG.md

@@ -13,6 +13,9 @@ each change under the current top section as you work.
 
 ## [1.5.5]
 
+### Changed
+- **Payments now run on the current Stripe library.** The plugin had fallen four major versions behind Stripe's PHP SDK, which also meant it was asking Stripe to behave like an older version of its API. Taking a card payment and handling a webhook work exactly as before — the same charge is raised, the same events are honoured, and a forged webhook is still rejected — but the plugin now speaks Stripe's current API version, so later payment work does not have to clear this upgrade first. Nothing to do on your side.
+
 ## [1.5.4]
 
 ### Fixed

composer.json

@@ -5,7 +5,7 @@
     "license": "GPL-2.0-or-later",
     "require": {
         "php": ">=8.1",
-        "stripe/stripe-php": "^17.0"
+        "stripe/stripe-php": "^21.0"
     },
     "require-dev": {
         "phpunit/phpunit": "^10.5",

composer.lock

@@ -4,32 +4,32 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "24094d05e029ee5534769a89f1dbad12",
+    "content-hash": "015f4bbb03f15362596d3a14aeca1582",
     "packages": [
         {
             "name": "stripe/stripe-php",
-            "version": "v17.6.0",
+            "version": "v21.2.1",
             "source": {
                 "type": "git",
                 "url": "https://github.com/stripe/stripe-php.git",
-                "reference": "a6219df5df1324a0d3f1da25fb5e4b8a3307ea16"
+                "reference": "da798f6224c5ff6f89d8d78025044f418ee27db8"
             },
             "dist": {
                 "type": "zip",
-                "url": "https://api.github.com/repos/stripe/stripe-php/zipball/a6219df5df1324a0d3f1da25fb5e4b8a3307ea16",
-                "reference": "a6219df5df1324a0d3f1da25fb5e4b8a3307ea16",
+                "url": "https://api.github.com/repos/stripe/stripe-php/zipball/da798f6224c5ff6f89d8d78025044f418ee27db8",
+                "reference": "da798f6224c5ff6f89d8d78025044f418ee27db8",
                 "shasum": ""
             },
             "require": {
                 "ext-curl": "*",
                 "ext-json": "*",
                 "ext-mbstring": "*",
-                "php": ">=5.6.0"
+                "php": ">=7.2.0"
             },
             "require-dev": {
-                "friendsofphp/php-cs-fixer": "3.72.0",
+                "friendsofphp/php-cs-fixer": "3.94.0",
                 "phpstan/phpstan": "^1.2",
-                "phpunit/phpunit": "^5.7 || ^9.0"
+                "phpunit/phpunit": "^8.0 || ^9.0"
             },
             "type": "library",
             "extra": {
@@ -38,6 +38,9 @@
                 }
             },
             "autoload": {
+                "files": [
+                    "lib/version_check.php"
+                ],
                 "psr-4": {
                     "Stripe\\": "lib/"
                 }
@@ -61,9 +64,9 @@
             ],
             "support": {
                 "issues": "https://github.com/stripe/stripe-php/issues",
-                "source": "https://github.com/stripe/stripe-php/tree/v17.6.0"
+                "source": "https://github.com/stripe/stripe-php/tree/v21.2.1"
             },
-            "time": "2025-08-27T19:32:42+00:00"
+            "time": "2026-08-20T21:06:13+00:00"
         }
     ],
     "packages-dev": [