CI: Validate on Ruby 2.0

Author: amotl

.github/workflows/tests.yml

@@ -25,14 +25,7 @@ jobs:
       matrix:
         os: [ "ubuntu-22.04", "macos-15-intel" ] # , windows-latest ]
         ruby-version: [
-          "2.4",
-          "2.5",
-          "2.6",
-          "2.7",
-          "3.0",
-          "3.1",
-          "3.2",
-          "3.3",
+          "2.0",
           "3.4",
         ]
 
@@ -74,6 +67,6 @@ jobs:
         run: bundle exec rspec
 
       - name: Run rubocop
-        # rubocop does not work on old versions of Ruby.
-        if: ${{ matrix.ruby-version != '2.4' && matrix.ruby-version != '2.5' }}
+        # rubocop 1.39 does not work on Ruby up to 2.5.
+        if: ${{ matrix.ruby-version != '2.0' }}
         run: bundle exec rubocop

crate_ruby.gemspec

@@ -62,7 +62,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency 'rubocop-rspec'
 
   # Needed by rubocop.
-  spec.add_development_dependency 'base64'
+  spec.add_development_dependency 'base64' if RUBY_VERSION >= '2.3'
   spec.add_development_dependency 'benchmark'
   spec.add_development_dependency 'ostruct'
 end