chore(deps): switch sitemap_generator to rubygems

[mroderick]

Summary

Switch sitemap_generator from GitHub source to RubyGems for improved stability.

Changes

• Remove GitHub lock (was waiting for version > 6.3.0 to be released)
• Use RubyGems version 7.0.1 with ~> 7.0 constraint
• sitemap_generator 7.x includes Rails 8 support

Why

The dependency was locked to GitHub because we needed a version higher than 6.3.0, which wasn't available on RubyGems at the time. Version 7.0.1 is now available on RubyGems, allowing us to use the standard gem source.

Benefits:

• More stable dependency resolution (no GitHub API rate limits)
• Faster bundle installs (RubyGems is cached by bundler)
• Clear version constraint prevents unexpected major version upgrades

Verification

• bundle install succeeds
• bundle exec rake sitemap:refresh works correctly
• public/sitemap.xml.gz is generated successfully

Breaking Changes Assessment

sitemap_generator 7.0 has some breaking changes that don't affect this project:

Change	Impact
Default search engines list is empty	None - we don't use the ping feature
LinkSet#create finalize! behavior change	None - we use standard create with a block
Ruby 2.5 / Rails 5.2 dropped	None - we use Rails 8.1, Ruby 3.x

[olleolleolle]

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