Avoid more warnings when using RubyGems with old Bundler

We were only avoiding them when the RUBYGEMS_GEMDEPS variable is used.
Avoid the warnings in general, whenever the entrypoint to Bundler is
`require`.

Author: deivid-rodriguez

.github/workflows/install-rubygems.yml

@@ -81,6 +81,12 @@ jobs:
         run: bundle install --help | grep -q BUNDLE-INSTALL
       - name: Check bundler fallback man pages are properly picked up
         run: sudo rm $(which man) && bundle install --help
+      - name: Install older bundler
+        run: gem install bundler:2.6.9
+      - name: Run older bundler without warnings
+        run: |
+          BUNDLER_VERSION=2.6.9 ruby -rbundler/setup -e1 2> errors.txt || (cat errors.txt && exit 1)
+          test ! -s errors.txt || (cat errors.txt && exit 1)
       - name: Build bundler
         run: gem build bundler.gemspec
         working-directory: ./bundler

lib/rubygems.rb

@@ -640,9 +640,8 @@ def self.load_safe_marshal
   # Load Bundler extensions to RubyGems, making sure to avoid redefinition
   # warnings in platform constants
 
-  def self.load_bundler_extensions
-    require "bundler/version"
-    return if Bundler::VERSION >= "2.6.9"
+  def self.load_bundler_extensions(version)
+    return unless version <= "2.6.9"
 
     previous_platforms = {}
 
@@ -1169,7 +1168,6 @@ def self.use_gemdeps(path = nil)
 
     ENV["BUNDLE_GEMFILE"] ||= File.expand_path(path)
     require_relative "rubygems/user_interaction"
-    Gem.load_bundler_extensions
     require "bundler"
     begin
       Gem::DefaultUserInteraction.use_ui(ui) do

lib/rubygems/core_ext/kernel_require.rb

@@ -64,8 +64,11 @@ def require(path) # :doc:
           rp
         end
 
-        Kernel.send(:gem, name, Gem::Requirement.default_prerelease) unless
-          resolved_path
+        next if resolved_path
+
+        Kernel.send(:gem, name, Gem::Requirement.default_prerelease)
+
+        Gem.load_bundler_extensions(Gem.loaded_specs[name].version) if name == "bundler"
 
         next
       end