Revert "Use git version of minitest on Ruby 3.3"

This reverts commit 600b2c4.

This doesn't work because minitest does not include a gemspec
in the repository.

.ci.gemfile

@@ -1,30 +1,24 @@
 source 'http://rubygems.org'
 
-if RUBY_VERSION >= '3.3'
-  gem 'minitest', git: 'https://github.com/minitest/minitest'
+minitest_version = case RUBY_VERSION
+when /\A1.8/
+  '< 5.12'
+when /\A1.9/
+  '< 5.4'
+when /\A2.0/
+  '< 5.6'
+when /\A2.1/
+  '< 5.8'
+when /\A2.2/
+  '< 5.9'
+when /\A2.3/
+  '< 5.10'
+when /\A2.4/
+  '< 5.11'
 else
-  minitest_version = case RUBY_VERSION
-  when /\a1.8/
-    '< 5.12'
-  when /\A1.9/
-    '< 5.4'
-  when /\A2.0/
-    '< 5.6'
-  when /\A2.1/
-    '< 5.8'
-  when /\A2.2/
-    '< 5.9'
-  when /\A2.3/
-    '< 5.10'
-  when /\A2.4/
-    '< 5.11'
-  else
-    '> 0'
-  end
-
-  gem 'minitest', minitest_version
+  '> 0'
 end
-
+gem 'minitest', minitest_version
 gem 'minitest-global_expectations', '>=1.0.1'
 
 if RUBY_VERSION < '2.0'