4.0.0

Upgrade to PHP 8.0+ and replace SwiftMailer with Symfony Mailer

• Update minimum PHP version requirement from 7.4 to 8.0
• Replace swiftmailer/swiftmailer with symfony/mailer
• Add guzzlehttp/guzzle as explicit dependency
• Update PHPUnit to ^9.0|^10.0|^11.0
• Update Symfony components to ^5.0|^6.0|^7.0

Source code modernization:

• Add PHP 8 typed properties and return types throughout codebase
• Use union types (string|false, string|callable)
• Use nullable types (?Type) instead of = null
• Replace list() with array destructuring syntax
• Replace call_user_func() with direct invocation
• Use match expression for SMTP security scheme
• Fix loose comparisons (== to ===)

Test suite updates:

• Migrate from PHPUnit_Framework_TestCase to PHPUnit\Framework\TestCase
• Add :void return types to setUp/tearDown methods
• Replace deprecated getMock() with createMock()/createPartialMock()
• Replace deprecated assertInternalType() with assertIsArray()
• Replace deprecated assertContains() with assertStringContainsString()
• Replace @ExpectedException annotations with expectException() method
• Update mail tests to use Symfony MailerInterface