Add task to migrate LDAP group links to multiple LDAP setup

This still requires some additional documentation

lib/tasks/migrate/ldap.rake

+desc "GITLAB | migrate provider names to multiple ldap setup"
+namespace :gitlab do
+  task migrate_ldap_providers: :environment do
+    config = Gitlab::LDAP::Config
+    raise 'No LDAP server hash defined. See config/gitlab.yml.example for an example' unless config.servers.any?
+
+    provider = config.servers.first['provider_name']
+    valid_providers = config.providers
+    unmigrated_group_links = LdapGroupLink.where('provider IS NULL OR provider NOT IN (?)', config.providers)
+    puts "found #{unmigrated_group_links.count} unmigrated LDAP links"
+    puts "setting provider to #{provider}"
+    unmigrated_group_links.update_all(provider: provider)
+  end
+end