Enable rubygems_mfa_required for gemspec

Fixes the following RuboCop failure:

```
bundle exec rubocop -ESP
Inspecting 29 files
.........................C...

Offenses:

solidus_dev_support.gemspec:5:1: C: [Correctable] Gemspec/RequireMFA: metadata['rubygems_mfa_required'] must be set to 'true'. (https://guides.rubygems.org/mfa-requirement-opt-in/)
Gem::Specification.new do |spec| ...
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

29 files inspected, 1 offense detected, 1 offense auto-correctable
```

Author: gsmendoza

solidus_dev_support.gemspec

@@ -15,6 +15,7 @@ Gem::Specification.new do |spec|
   spec.metadata['homepage_uri'] = spec.homepage
   spec.metadata['source_code_uri'] = 'https://github.com/solidusio/solidus_dev_support'
   spec.metadata['changelog_uri'] = 'https://github.com/solidusio/solidus_dev_support/blob/master/CHANGELOG.md'
+  spec.metadata['rubygems_mfa_required'] = 'true'
 
   spec.required_ruby_version = '>= 2.5.0'