fix: quoted-printable unicode and newlines in mails

What does this MR do?

Patch for Mail gem fixing encoding issues when enclosing an existing email object in a new Mail object.

This affects particularly the SMIME emails flow, since the re-encoding changes will invalidate the calculated signatures.

See upstream issue: https://github.com/mikel/mail/issues/1190

Fixes #197386 (closed)

🛠 with ❤ at Siemens

/cc @bufferoverflow @rpaik

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Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
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• Code review guidelines
• Merge request performance guidelines
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• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
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