Bump PHP to 8.1 minimum and resolve composer.lock in CI

The Gitea release build failed because aws/aws-sdk-php 3.371+ requires PHP
>= 8.1, while composer.json's platform was pinned to 8.0. Production runs
PHP 8.5.4, so bumping the minimum to 8.1 is well within the runtime envelope.

Changes:
  composer.json                 — require php >=8.1, platform.php = 8.1
  .gitea/workflows/release.yml  — setup-php now installs PHP 8.1
                                  added pre-install step that runs
                                  `composer update --no-install` only when
                                  composer.lock is absent. If a lock file is
                                  ever committed later, CI uses it directly
                                  for deterministic installs instead of
                                  regenerating.
  twilio-wp-plugin.php          — added "Requires PHP: 8.1" header so
                                  WordPress itself enforces the minimum
  README.md, CLAUDE.md          — doc bump from 8.0 to 8.1, with note that
                                  the AWS SDK is the constraint driver

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

composer.json

@@ -3,7 +3,7 @@
     "description": "WordPress plugin for Twilio voice and SMS integration - REQUIRES Twilio SDK",
     "type": "wordpress-plugin",
     "require": {
-        "php": ">=8.0",
+        "php": ">=8.1",
         "twilio/sdk": "^8.7",
         "aws/aws-sdk-php": "^3.0"
     },
@@ -19,7 +19,7 @@
     "config": {
         "optimize-autoloader": true,
         "platform": {
-            "php": "8.0"
+            "php": "8.1"
         }
     },
     "scripts": {