We officially support Ruby >= 2.7.5

Adding .ruby-version made rubocop want to enforce 3.1 syntax, which isn't
what we said we'd support in the gemspec (we said 2.3 (!)) - bump that
to 2.7, which is the earliest minor version any of our live services use

.rubocop.yml

@@ -42,6 +42,7 @@ Style/EachWithObject:
   Enabled: false
 
 AllCops:
+  TargetRubyVersion: 2.7
   NewCops: enable
   Exclude:
     - 'spec/dummy/db/schema.rb'

dfe-analytics.gemspec

@@ -11,7 +11,7 @@ Gem::Specification.new do |spec|
   spec.summary       = 'Event pump for DfE Rails applications'
   spec.homepage      = 'https://teacher-services-tech-docs.london.cloudapps.digital/#teacher-services-technical-documentation'
   spec.license       = 'MIT'
-  spec.required_ruby_version = Gem::Requirement.new('>= 2.3.0')
+  spec.required_ruby_version = Gem::Requirement.new('>= 2.7.5')
 
   spec.metadata['allowed_push_host'] = "TODO: Set to 'http://mygemserver.com'"