[ruby/rubygems] Fix Bundler crashing when installing very old gems:

- ### Problem

  There is a regression where bundler can longer install gems that
  were packaged a long time ago, like the [memoize gem](https://rubygems.org/gems/memoize).

  Bundler will crash with such error:

  ```
  Installing memoize 1.3.1
  --- ERROR REPORT TEMPLATE -------------------------------------------------------

  NoMethodError: undefined method 'as_list' for nil
  /Users/edouard/src/opensource/rubygems/lib/rubygems/dependency.rb:146:in 'Gem::Dependency#requirements_list'
  ```

  ### Context

  These very old gems were packaged at the time were a
  `Gem::Dependency` had no ivars `requirement`.
  This is what the dumped gemspec looks like:

  ```
  dependencies:
  - !ruby/object:Gem::Dependency
  name: test-unit
  type: :development
  version_requirement:
  version_requirements: !ruby/object:Gem::Requirement
    requirements:
    - - ">="
      - !ruby/object:Gem::Version
        version: 2.0.2
    version:
  ```

  As opposed to a gem that was packaged more recently:

  ```
  - !ruby/object:Gem::Dependency
  name: minitest-global_expectations
  requirement: !ruby/object:Gem::Requirement
    requirements:
    - - ">="
      - !ruby/object:Gem::Version
        version: '0'
  type: :development
  prerelease: false
  version_requirements: !ruby/object:Gem::Requirement
    requirements:
    - - ">="
      - !ruby/object:Gem::Version
        version: '0'
  ```

  The change we made in ruby/rubygems@cbe57bb#diff-5ccc8aa468df013cc970580e88642417da12284da6529a4885b9abf447ae95edL529
  broke old gem installation.

  ### Solution

  If `requirement` isn't set in the hash, look for
  `version_requirements` instead.

ruby/rubygems@cfd8b20692

Author: Edouard-chin

lib/rubygems/yaml_serializer.rb

@@ -573,7 +573,7 @@ def build_dependency(node)
         d = Gem::Dependency.allocate
         d.instance_variable_set(:@name, hash["name"])
 
-        d.instance_variable_set(:@requirement, hash["requirement"])
+        d.instance_variable_set(:@requirement, hash["requirement"] || hash["version_requirements"])
 
         type = hash["type"]
         type = type ? type.to_s.sub(/^:/, "").to_sym : :runtime

test/rubygems/test_gem_safe_yaml.rb

@@ -921,6 +921,26 @@ def test_load_version_requirement_old_tag
     assert_equal [[">=", Gem::Version.new("1.0")]], req.requirements
   end
 
+  def test_load_dependency_version_version_requirement_old_tag
+    yaml = <<~YAML
+      - !ruby/object:Gem::Dependency
+        name: test-unit
+        type: :development
+        version_requirement:
+        version_requirements: !ruby/object:Gem::Requirement
+          requirements:
+          - - ">="
+            - !ruby/object:Gem::Version
+              version: 2.0.2
+          version:
+    YAML
+
+    deps = yaml_load(yaml, permitted_classes: Gem::SafeYAML::PERMITTED_CLASSES)
+    assert_not_nil(deps.first)
+
+    assert_equal [[">=", Gem::Version.new("2.0.2")]], deps.first.requirement.requirements
+  end
+
   def test_load_platform_from_value_field
     yaml = "!ruby/object:Gem::Platform\nvalue: x86-linux\n"
     plat = yaml_load(yaml, permitted_classes: Gem::SafeYAML::PERMITTED_CLASSES)