Update dependency nodemailer to v6

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
nodemailer (source)	dependencies	major	5.1.1 -> 6.3.0

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Release Notes

nodemailer/nodemailer

v6.3.0

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• Added new option to pass a set of httpHeaders to be sent when fetching attachments. See PR #​1034

v6.2.1

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• No changes. It is the same as 6.2.0 that was accidentally published as 6.2.1 to npm

v6.1.1

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• Fixed regression bug with missing smtp authMethod property

v6.1.0

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• Added new message property amp for providing AMP4EMAIL content

v6.0.0

Compare Source

• SMTPConnection: use removeListener instead of removeAllListeners (xr0master) [ddc4af15]
  Using removeListener should fix memory leak with Node.js streams

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