Removes dev client from security tasks

What does this MR do?

Omnibus is currently moving its security development to GitLab Security, so it won't be longer necessary to validate and merge MR on dev.gitlab.org.

Also modifies the MergeRequestsValidator#projects_to_verify to return only GitLab Security and Omnibus GitLab Security

Related to gitlab-com/gl-infra/delivery#692 (closed)

lib/release_tools/security/merge_requests_validator.rb

@@ -6,6 +6,11 @@ module ReleaseTools
     class MergeRequestsValidator
       include ::SemanticLogger::Loggable
 
+      PROJECTS_TO_VERIFY = %w[
+        gitlab-org/security/gitlab
+        gitlab-org/security/omnibus-gitlab
+      ].freeze
+
       ERROR_FOOTNOTE = <<~FOOTNOTE.strip
         <hr>
 
@@ -57,7 +62,7 @@ module ReleaseTools
         valid = []
         invalid = []
 
-        projects_to_verify.each do |project|
+        PROJECTS_TO_VERIFY.each do |project|
           logger.info('Verifying security MRs', project: project)
 
           merge_requests = @client.open_security_merge_requests(project)
@@ -126,7+131,7 @@
 
       private
 
       def projects_to_verify
         %w[
           gitlab-org/security/gitlab
           gitlab-org/security/omnibus-gitlab