build(deps): update gollum requirement from 5.1.2 to 5.2.1 (#7)

Updates the requirements on [gollum](https://github.com/gollum/gollum) to permit the latest version.
- [Release notes](https://github.com/gollum/gollum/releases)
- [Changelog](https://github.com/gollum/gollum/blob/master/HISTORY.md)
- [Commits](gollum/gollum@v5.1.2...v5.2.1)

Signed-off-by: dependabot[bot] <support@github.com>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Gemfile

@@ -1,3 +1,3 @@
 source "https://rubygems.org"
 
-gem 'gollum', '5.1.2'
+gem 'gollum', '5.2.1'