Bump github.com/peterbourgon/ff/v3 from 3.3.2 to 3.4.0

Bumps [github.com/peterbourgon/ff/v3](https://github.com/peterbourgon/ff) from 3.3.2 to 3.4.0.
- [Release notes](https://github.com/peterbourgon/ff/releases)
- [Commits](https://github.com/peterbourgon/ff/compare/v3.3.2...v3.4.0)

---
updated-dependencies:
- dependency-name: github.com/peterbourgon/ff/v3
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

go.mod

@@ -3,7 +3,7 @@ module github.com/decke/smtprelay
 require (
 	github.com/chrj/smtpd v0.3.1
 	github.com/google/uuid v1.3.0
-	github.com/peterbourgon/ff/v3 v3.3.2
+	github.com/peterbourgon/ff/v3 v3.4.0
 	github.com/sirupsen/logrus v1.9.3
 	github.com/stretchr/testify v1.8.4
 	golang.org/x/crypto v0.11.0