fix(deps): update module github.com/mailgun/mailgun-go/v4 to v4.18.0

[renovate[bot]]

This PR contains the following updates:

Package	Change	Age	Adoption	Passing	Confidence
github.com/mailgun/mailgun-go/v4	v4.17.3 -> v4.18.0

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Release Notes

mailgun/mailgun-go (github.com/mailgun/mailgun-go/v4)

### [`v4.18.0`](https://redirect.github.com/mailgun/mailgun-go/releases/tag/v4.18.0) [Compare Source](https://redirect.github.com/mailgun/mailgun-go/compare/v4.17.3...v4.18.0) #### What's Changed - DE-1343 Create func NewMessage and NewMIMEMessage by [@​vtopc](https://redirect.github.com/vtopc) in [https://github.com/mailgun/mailgun-go/pull/335](https://redirect.github.com/mailgun/mailgun-go/pull/335) **Full Changelog**: https://github.com/mailgun/mailgun-go/compare/v4.17.3...v4.18.0

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