Fix dependency warnings (#176)

warning: ostruct was loaded from the standard library, but will no longer
be part of the default gems starting from Ruby 3.5.0.

warning: bigdecimal was loaded from the standard library, but will no longer
be part of the default gems starting from Ruby 3.4.0.

Requires Ruby >= 2.5.0. Resolves GH-175.

.github/workflows/build.yml

@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        ruby-version: ['2.3', '2.4', '2.5', '2.6', '2.7', '3.0']
+        ruby-version: ['2.5', '2.6', '2.7', '3.0', '3.1', '3.2', '3.3']
 
     name: Ruby (${{ matrix.ruby-version }})
     steps:

mollie-api-ruby.gemspec

@@ -20,11 +20,14 @@ Gem::Specification.new do |s|
     'changelog_uri' => 'https://github.com/mollie/mollie-api-ruby/blob/master/CHANGELOG.md'
   }
 
-  s.required_ruby_version = '>= 2.3.8'
+  s.required_ruby_version = '>= 2.5'
 
   s.files = `git ls-files`.split("\n")
   s.test_files = Dir['test/**/*']
 
+  s.add_dependency('ostruct', '~> 0.6.0')
+  s.add_dependency('bigdecimal', '~> 3.1', '>= 3.1.8')
+
   s.add_development_dependency('rake')
   s.add_development_dependency('rubocop', '~> 0.57.2')
   s.add_development_dependency('test-unit')