chore(deps): update axllent/mailpit docker tag to v1.20.4 (stable/8.5)

[renovate[bot]]

This PR contains the following updates:

Package	Update	Change
axllent/mailpit (source)	patch	v1.20.3 -> v1.20.4

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Release Notes

axllent/mailpit (axllent/mailpit)

### [`v1.20.4`](https://redirect.github.com/axllent/mailpit/blob/HEAD/CHANGELOG.md#v1204) [Compare Source](https://redirect.github.com/axllent/mailpit/compare/v1.20.3...v1.20.4) ##### Chore - Update Go modules - Update node modules - Upgrade vue-css-donut-chart & related charts ##### Fix - Relax URL detection in link check tool ([#​357](https://redirect.github.com/axllent/mailpit/issues/357))

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