Add upgrade guide (#387)

* Create UPGRADE.md

* Update UPGRADE.md

Co-authored-by: Taylor Otwell <taylor@laravel.com>

UPGRADE.md

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+# Upgrade Guide
+
+## Upgrading To 3.0 From 2.x
+
+### Minimum Versions
+
+The following dependency versions have been updated:
+
+- The minimum PHP version is now v8.0.2
+- The minimum Laravel version is now v9.21
+
+### New `expired_at` Column
+
+Sanctum now supports expiring tokens. To support this feature, a new `expired_at` column must be added to your application's `personal_access_tokens` table. To add the column to your table, create a migration with the following schema change:
+
+```php
+Schema::table('personal_access_tokens', function (Blueprint $table) {
+    $table->timestamp('expires_at')->nullable()->after('last_used_at');
+});
+```
+
+Running this migration will require you to [install the `doctrine/dbal` package](https://laravel.com/docs/migrations#renaming-columns).