chore(deps): bump github.com/emersion/go-smtp from 0.16.0 to 0.18.1 (#1771)

Bumps [github.com/emersion/go-smtp](https://github.com/emersion/go-smtp) from 0.16.0 to 0.18.1.
- [Release notes](https://github.com/emersion/go-smtp/releases)
- [Commits](https://github.com/emersion/go-smtp/compare/v0.16.0...v0.18.1)

---
updated-dependencies:
- dependency-name: github.com/emersion/go-smtp
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>
Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

go.mod

@@ -17,7 +17,7 @@ require (
 	github.com/cenkalti/backoff v2.2.1+incompatible
 	github.com/d4l3k/messagediff v1.2.2-0.20190829033028-7e0a312ae40b
 	github.com/emersion/go-sasl v0.0.0-20200509203442-7bfe0ed36a21
-	github.com/emersion/go-smtp v0.16.0
+	github.com/emersion/go-smtp v0.18.1
 	github.com/google/go-cmp v0.6.0
 	github.com/google/subcommands v1.2.0
 	github.com/google/uuid v1.4.0