fix: Downgrade json_schemer

This fixes:

bundle
Fetching gem metadata from https://rubygems.org/.......
Resolving dependencies...
Could not find compatible versions

Because gitlab-labkit >= 1.1.0 depends on json_schemer ~> 2.4.0
  and Gemfile depends on gitlab-labkit ~> 1.1.1,
  json_schemer ~> 2.4.0 is required.
So, because Gemfile depends on json_schemer ~> 2.3.0,
  version solving has failed.

Trying to upgrade json_schemer in rails led to:

bundle
Fetching gem metadata from https://rubygems.org/.......
Resolving dependencies...
Could not find compatible versions

Because gitlab-security_report_schemas >= 0.1.1.min15.0.0.max15.1.4, < 0.2.0.min15.0.0.max15.2.3 depends on json_schemer ~> 2.3.0
  and Gemfile depends on gitlab-security_report_schemas = 0.1.3.min15.0.0.max15.2.3,
  json_schemer ~> 2.3.0 is required.
So, because Gemfile depends on json_schemer ~> 2.4.0,
  version solving has failed.

So, let's keep match the version.

I've tried this locally on the rails app and it works:

diff --git a/Gemfile b/Gemfile
index 398f9862d11d..7357cd90bb9a 100644
--- a/Gemfile
+++ b/Gemfile
@@ -400,7 +400,8 @@ gem 'gitlab-schema-validation', path: 'gems/gitlab-schema-validation', feature_c
 gem 'gitlab-http', path: 'gems/gitlab-http', feature_category: :shared # rubocop:todo Gemfile/MissingFeatureCategory -- https://gitlab.com/gitlab-org/gitlab/-/issues/581839

 gem 'premailer-rails', '~> 1.12.0', feature_category: :notifications
-gem 'gitlab-labkit', '~> 1.0.0', feature_category: :shared # rubocop:todo Gemfile/MissingFeatureCategory -- https://gitlab.com/gitlab-org/gitlab/-/issues/581839
+# gem 'gitlab-labkit', '~> 1.0.0', feature_category: :shared # rubocop:todo Gemfile/MissingFeatureCategory -- https://gitlab.com/gitlab-org/gitlab/-/issues/581839
+gem 'gitlab-labkit', feature_category: :shared, path: '/Users/hercules/GitLab/labkit-ruby', ref: '421cfc6'
 gem 'thrift', '~> 0.22.0', feature_category: :shared # rubocop:todo Gemfile/MissingFeatureCategory -- https://gitlab.com/gitlab-org/gitlab/-/issues/581839

 # I18n