External Libraries: Upgrade PHPMailer to version 7.0.0.

This is exactly the same as version 6.11.1 but bumps the major version number to indicate the presence of a backward compatibility break in child classes.

Specifically, `lang()`, `setLanguage()`, and `$language` are now static, and should be called statically.

References:
* [https://github.com/PHPMailer/PHPMailer/releases/tag/v7.0.0 PHPMailer 7.0.0 release notes]
* [PHPMailer/PHPMailer@v6.11.1...v7.0.0 Full list of changes in PHPMailer 7.0.0]

Follow-up to [54937], [55557], [56484], [57137], [59246], [59481], [60623], [60813], [60888].

Props SirLouen.
Fixes #64101.

git-svn-id: https://develop.svn.wordpress.org/trunk@61249 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-includes/PHPMailer/PHPMailer.php

@@ -768,7 +768,7 @@ class PHPMailer
      *
      * @var string
      */
-    const VERSION = '6.11.1';
+    const VERSION = '7.0.0';
 
     /**
      * Error severity: message only, continue processing.

src/wp-includes/PHPMailer/POP3.php

@@ -46,7 +46,7 @@ class POP3
      *
      * @var string
      */
-    const VERSION = '6.11.1';
+    const VERSION = '7.0.0';
 
     /**
      * Default POP3 port number.

src/wp-includes/PHPMailer/SMTP.php

@@ -35,7 +35,7 @@ class SMTP
      *
      * @var string
      */
-    const VERSION = '6.11.1';
+    const VERSION = '7.0.0';
 
     /**
      * SMTP line break constant.