Bump phpmailer/phpmailer from 6.10.0 to 6.11.1

Bumps phpmailer/phpmailer from 6.10.0 to 6.11.1.

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updated-dependencies:

• dependency-name: phpmailer/phpmailer
  dependency-version: 6.11.1
  dependency-type: direct:production
  update-type: version-update:semver-minor
  ...

Fixes 0035874, https://github.com/mantisbt/mantisbt/pull/2152

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