Update net-smtp requirement from ~> 0.3.1 to ~> 0.4.0

Updates the requirements on [net-smtp](https://github.com/ruby/net-smtp) to permit the latest version.
- [Release notes](https://github.com/ruby/net-smtp/releases)
- [Changelog](https://github.com/ruby/net-smtp/blob/master/NEWS.md)
- [Commits](ruby/net-smtp@v0.3.1...v0.4.0)

---
updated-dependencies:
- dependency-name: net-smtp
  dependency-type: direct:production
...

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

Gemfile

@@ -35,7 +35,7 @@ gem "sprockets", "~> 4.0"
 # Allow throttling requests
 gem "rack-attack", "~> 6.5"
 
-gem "net-smtp", "~> 0.3.1"
+gem "net-smtp", "~> 0.4.0"
 gem "psych", "~> 5.1"
 
 group :development, :test do