Bump phpmailer/phpmailer from 6.6.3 to 6.6.4

Bumps phpmailer/phpmailer from 6.6.3 to 6.6.4.

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updated-dependencies:

• dependency-name: phpmailer/phpmailer
  dependency-type: direct:production
  update-type: version-update:semver-patch
  ...

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Fixes 0029025, PR https://github.com/mantisbt/mantisbt/pull/1845