Update json gem to v2.5.1

In preparation for the Ruby 3 upgrade, we should update
json to match the minimum gem version as specified by
the default gem:

https://stdgems.org/json/#gem-version

Which for Ruby 3.0 is 2.5.1.

This will otherwise lead to errors, since rubygems
will encounter conflicting gemspecs during require.

See https://gitlab.com/gitlab-org/omnibus-gitlab/-/issues/6286

Gemfile

@@ -522,7 +522,7 @@ gem 'lockbox', '~> 0.6.2'
 gem 'valid_email', '~> 0.1'
 
 # JSON
-gem 'json', '~> 2.3.0'
+gem 'json', '~> 2.5.1'
 gem 'json_schemer', '~> 0.2.18'
 gem 'oj', '~> 3.10.6'
 gem 'multi_json', '~> 1.14.1'

Gemfile.lock

@@ -657,7 +657,7 @@ GEM
       character_set (~> 1.4)
       regexp_parser (~> 2.1)
       regexp_property_values (~> 1.0)
-    json (2.3.0)
+    json (2.5.1)
     json-jwt (1.13.0)
       activesupport (>= 4.2)
       aes_key_wrap
@@ -1509,7 +1509,7 @@ DEPENDENCIES
   ipaddress (~> 0.8.3)
   jira-ruby (~> 2.1.4)
   js_regex (~> 3.7)
-  json (~> 2.3.0)
+  json (~> 2.5.1)
   json_schemer (~> 0.2.18)
   jwt (~> 2.1.0)
   kaminari (~> 1.0)