Update dependency phpmailer/phpmailer to v6.6.3

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
phpmailer/phpmailer	require	patch	6.6.2 -> 6.6.3

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Release Notes

PHPMailer/PHPMailer

v6.6.3

Compare Source

• Add an HTML form to the OAuth setup script
• Minor CS improvements
• Add Mongolian translation
• Remove bogus "ch" translation

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