Bump github.com/mailgun/mailgun-go/v4 from 4.20.5 to 4.21.0

Bumps [github.com/mailgun/mailgun-go/v4](https://github.com/mailgun/mailgun-go) from 4.20.5 to 4.21.0.
- [Release notes](https://github.com/mailgun/mailgun-go/releases)
- [Changelog](https://github.com/mailgun/mailgun-go/blob/master/CHANGELOG)
- [Commits](mailgun/mailgun-go@v4.20.5...v4.21.0)

---
updated-dependencies:
- dependency-name: github.com/mailgun/mailgun-go/v4
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

go.mod

@@ -9,7 +9,7 @@ require (
 	github.com/go-openapi/swag v0.23.0
 	github.com/google/certificate-transparency-go v1.3.0
 	github.com/google/go-github/v65 v65.0.0
-	github.com/mailgun/mailgun-go/v4 v4.20.5
+	github.com/mailgun/mailgun-go/v4 v4.21.0
 	github.com/migueleliasweb/go-github-mock v1.1.0
 	github.com/mocktools/go-smtp-mock/v2 v2.4.0
 	github.com/sendgrid/sendgrid-go v3.16.0+incompatible

go.sum

@@ -133,8 +133,8 @@ github.com/magiconair/properties v1.8.7 h1:IeQXZAiQcpL9mgcAe1Nu6cX9LLw6ExEHKjN0V
 github.com/magiconair/properties v1.8.7/go.mod h1:Dhd985XPs7jluiymwWYZ0G4Z61jb3vdS329zhj2hYo0=
 github.com/mailgun/errors v0.4.0 h1:6LFBvod6VIW83CMIOT9sYNp28TCX0NejFPP4dSX++i8=
 github.com/mailgun/errors v0.4.0/go.mod h1:xGBaaKdEdQT0/FhwvoXv4oBaqqmVZz9P1XEnvD/onc0=
-github.com/mailgun/mailgun-go/v4 v4.20.5 h1:X0n0faLwHy8iQmCVrzOmJAwsAEGjvtcCu38patBUfbc=
-github.com/mailgun/mailgun-go/v4 v4.20.5/go.mod h1:768NjUvsxW8Ga8fHIPITmr5f/U8qmnQqnZ3/cs3StUc=
+github.com/mailgun/mailgun-go/v4 v4.21.0 h1:l9SvJDdFvQxB5J1/jCz6g3GELKJ5k2iCelShDuphv94=
+github.com/mailgun/mailgun-go/v4 v4.21.0/go.mod h1:768NjUvsxW8Ga8fHIPITmr5f/U8qmnQqnZ3/cs3StUc=
 github.com/mailru/easyjson v0.7.7 h1:UGYAvKxe3sBsEDzO8ZeWOSlIQfWFlxbzLZe7hwFURr0=
 github.com/mailru/easyjson v0.7.7/go.mod h1:xzfreul335JAWq5oZzymOObrkdz5UnU4kGfJJLY9Nlc=
 github.com/mattn/go-colorable v0.1.13 h1:fFA4WZxdEF4tXPZVKMLwD8oUnCTTo08duU7wxecdEvA=