Apply recaptcha API changes in 4.0

In recaptcha 4.0.0 there was an API change:

 - `public_key` -> `site_key`
 - `private_key` -> secret_key

See:
https://github.com/ambethia/recaptcha/blob/master/CHANGELOG.md

Gemfile.lock

@@ -680,7 +680,7 @@ GEM
       optimist (>= 3.0.0)
     rdoc (6.0.4)
     re2 (1.1.1)
-    recaptcha (4.13.0)
+    recaptcha (4.13.1)
       json
     recursive-open-struct (1.1.0)
     redis (3.3.5)
@@ -1118,7 +1118,6 @@ DEPENDENCIES
   rdoc (~> 6.0)
   re2 (~> 1.1.1)
   recaptcha (~> 4.11)
-  redcarpet (~> 3.4)
   redis (~> 3.2)
   redis-namespace (~> 1.6.0)
   redis-rails (~> 5.0.2)

changelogs/unreleased/pravi-gitlab-ce-update-recaptcha.yml

+---
+title: Apply recaptcha API change in 4.0
+merge_request: 25921
+author: Praveen Arimbrathodiyil
+type: other

lib/gitlab/recaptcha.rb

@@ -5,8 +5,8 @@ module Recaptcha
     def self.load_configurations!
       if Gitlab::CurrentSettings.recaptcha_enabled
         ::Recaptcha.configure do |config|
-          config.public_key  = Gitlab::CurrentSettings.recaptcha_site_key
-          config.private_key = Gitlab::CurrentSettings.recaptcha_private_key
+          config.site_key = Gitlab::CurrentSettings.recaptcha_site_key
+          config.secret_key = Gitlab::CurrentSettings.recaptcha_private_key
         end
 
         true