feat: Update gem dependencies and sitemap generator

[echarp]

This commit updates several gem dependencies in the Gemfile and Gemfile.lock, including Rails and related gems. The version constraint for the sitemap_generator gem has been removed, allowing the latest version to be used.

The sitemap configuration has been updated to match the new usage of sitemap_generator. The encoding comment and an extra line have been removed, and the add_links method has been replaced with create.

Extra lines have been added before sitemap.add calls for readability.