CI: update Ruby test matrix

- Remove v2.7: rubocop >= 1.51.0 depends on Ruby >= 2.7.0
- Add modern Ruby versions

Author: sgerrand

.github/workflows/ci.yaml

@@ -103,10 +103,11 @@ jobs:
       fail-fast: false
       matrix:
         ruby:
-          - ruby-2.6
           - ruby-2.7
           - ruby-3.0
           - ruby-3.1
+          - ruby-3.2
+          - ruby-3.3
 
     steps:
       - uses: actions/checkout@v3

duffel_api.gemspec

@@ -11,7 +11,7 @@ Gem::Specification.new do |spec|
   spec.summary       = "A Ruby client for interacting with the Duffel API"
   spec.homepage      = "https://github.com/duffelhq/duffel-api-ruby"
   spec.license       = "MIT"
-  spec.required_ruby_version = ">= 2.6.0"
+  spec.required_ruby_version = ">= 2.7.0"
 
   spec.metadata["homepage_uri"] = spec.homepage
   spec.metadata["source_code_uri"] = spec.homepage