WP Mail SMTP Pro 4.2.0

### Fixed:
- PHP 8 compatibility when existing Gmail mailer connection is revoked.

### Added:
- New transactional mailer: SparkPost integration.
- One-click migration from FluentSMTP plugin.
- Plugin constants integration in Setup Wizard.

### Fixed:
- Early plugin deactivation issue with activity log plugins.

### Added:
- New transactional mailer: Postmark integration.
- Email Log: added CC and BCC columns to the table.
- Support for string attachments (added via PHPMailer object).

### Changed:
- Improved Weekly Email Summary Setup Wizard control.
- Improved Email Source detection in Debug Events and Email Log for WP Core sent emails.
- Improved uninstall process. It now removes all plugin DB data and tables.

### Fixed:
- Email Source detection in Debug Events and Email Log for file paths with backslashes.
- Amazon SES automatic closest region detection (keycdn API request).
- Added SSL check for Amazon SES and Outlook mailers in the Setup Wizard.
- Blurry image assets in Weekly Email Summary.
- PHP extension mb_strings not polyfilled correctly.
- New user registration email template for Email Controls feature.
- Added missing is_email_sent filters for Sendinblue, Mailgun, and Gmail mailers.
- Debug Events double-entry DB save, because of a bug in is_email_sent method for certain mailers.
- Network subsite switcher in WP Multisite installations with subdomain configuration.