Preparing v5.2.0

[donv]

• Updated version to 5.2.0 in lib/bootstrap_form/version.rb
• Added running the demo tests in GitHub Actions.
• Updated CHANGELOG.md with link to GitHub releases and diff.
• Updated README.md with new versions and fresh examples and screenshots using Bootstrap 5.2.
• Updated RELEASING.md with running the tests and demo tests.
• Cleanup of demo/Gemfile for better readability.
• Updated demo/Gemfile.lock.
• Removed webpacker from demo/package.json. This reduced demo/yarn.lock from 7796 lines to 317.