Fix Bundler version compatibility for all Ruby versions

Use conditional Bundler version selection:
- Bundler 2.5.21 for Ruby 3.0+ (compatible with RubyGems 3.5.21)
- Bundler 2.3.26 for Ruby 2.5-2.7 (maintains existing compatibility)

This resolves the 'already initialized constant Gem::Platform' warnings
on newer Ruby versions while ensuring older Ruby versions continue to work.

Author: ericfirth

.github/workflows/test-macos.yaml

@@ -47,7 +47,7 @@ jobs:
         with:
           ruby-version: ${{ matrix.ruby }}
           rubygems: 3.3.26
-          bundler: 2.5.21 # updated for compatibility with RubyGems 3.5.21
+          bundler: ${{ matrix.ruby >= '3.0' && '2.5.21' || '2.3.26' }} # 2.5.21 for Ruby 3.0+, 2.3.26 for older versions
       # Specify gem version for 3.4 because default version (3.6.0.dev)
       # leads to an incorrect gem root path
       - if: ${{ matrix.ruby == '3.3' || matrix.ruby == '3.4' }}