Remove Github Changelog Generator dependency

Unfortunately this gem no longer works on our repository because it
reaches a request limit:

Octokit::TooManyRequests
403 - You have exceeded a secondary rate limit. Please wait a few
minutes before you try again.
See: https://docs.github.com/en/free-pro-team@latest/rest/overview/resources-in-the-rest-api#secondary-rate-limits

We can still use github_fast_changelog, which can be run locally without
being included as a dependencly, to help us update our CHANGELOG.

Gemfile.lock

@@ -244,8 +244,6 @@ GEM
     faraday-em_http (1.0.0)
     faraday-em_synchrony (1.0.0)
     faraday-excon (1.1.0)
-    faraday-http-cache (2.2.0)
-      faraday (>= 0.8)
     faraday-httpclient (1.0.1)
     faraday-multipart (1.0.4)
       multipart-post (~> 2)
@@ -269,14 +267,6 @@ GEM
       activemodel (>= 4.1, < 7.1)
       activesupport (>= 4.1, < 7.1)
       railties (>= 4.1, < 7.1)
-    github_changelog_generator (1.15.2)
-      activesupport
-      faraday-http-cache
-      multi_json
-      octokit (~> 4.6)
-      rainbow (>= 2.2.1)
-      rake (>= 10.0)
-      retriable (~> 3.0)
     gitlab (4.19.0)
       httparty (~> 0.20)
       terminal-table (>= 1.5.1)
@@ -525,7 +515,6 @@ GEM
     responders (3.0.1)
       actionpack (>= 5.0)
       railties (>= 5.0)
-    retriable (3.1.2)
     rexml (3.2.5)
     rinku (2.0.6)
     ros-apartment (2.11.0)
@@ -739,7 +728,6 @@ DEPENDENCIES
   font-awesome-sass (~> 5.15.1)
   foundation-rails (~> 6.6.2.0)
   foundation_rails_helper (~> 4.0.1)
-  github_changelog_generator (~> 1.15.2)
   globalize (~> 6.2.1)
   globalize-accessors (~> 0.3.0)
   graphiql-rails (~> 1.8.0)