Add Rails 8 compatibility

[codegen-sh]

This PR adds Rails 8 compatibility to the twitter-bootstrap-rails gem.

Changes:

1. Updated gemspec to support Rails 8 by changing dependency constraints:
   • Updated railties dependency to '>= 5.0', '< 9.0'
   • Updated actionpack dependency to '>= 5.0', '< 9.0'
2. Updated version to 5.1.0
3. Updated README and CHANGELOG to reflect Rails 8 support
4. Updated summary and description in gemspec to include Rails 8

These changes ensure the gem works with Rails 8 while maintaining backward compatibility with Rails 5, 6, and 7.