Fixup rubygems: latest to work with Rubies < 2.3

.github/workflows/test.yml

@@ -181,6 +181,7 @@ jobs:
         - { ruby: '2.6', expected_rubygems_version: '3.4.22' }
         - { ruby: '2.5', expected_rubygems_version: '3.3.27' }
         - { ruby: '2.3', expected_rubygems_version: '3.3.27' }
+        - { ruby: '2.0', expected_rubygems_version: '2.7.11' }
     steps:
     - uses: actions/checkout@v4
     - uses: ./

action.yml

@@ -13,7 +13,7 @@ inputs:
       The version of RubyGems to use. Either 'default' (the default), 'latest', or a version number (e.g., 3.3.5).
       For 'default', no action is taken and the version of RubyGems that comes with Ruby by default is used.
       For 'latest', `gem update --system` is run to update to the latest compatible RubyGems version.
-      Ruby head/master builds and Ruby 2.2 and earlier will not be updated.
+      Ruby head/master builds will not be updated.
       Similarly, if a version number is given, `gem update --system <version>` is run to update to that version of RubyGems, as long as that version is newer than the one provided by default.
   bundler:
     description: |

rubygems.js

@@ -45,6 +45,9 @@ async function rubygemsLatest(gem, platform, engine, rubyVersion) {
       await exec.exec(gem, ['update', '--system', '3.4.22'])
     } else if (floatVersion >= 2.3) {
       await exec.exec(gem, ['update', '--system', '3.3.27'])
+    } else if (floatVersion >= 1.9) {
+      await exec.exec(`${gem} install rubygems-update -v 2.7.11 --no-document`)
+      await exec.exec('update_rubygems')
     } else {
       console.log(`Cannot update RubyGems for Ruby version ${rubyVersion}`)
     }