Remove cluster_image_scanning from approval_project_rules table

What does this MR do?

Remove cluster_image_scanning from the scanners column in the approval_project_rules table, backend and frontend.

Related issue: #337244 (closed)

$ SKIP_POST_DEPLOYMENT_MIGRATIONS=true bundle exec rake db:migrate

== 20210804202057 AddTmpIndexApprovalProjectRulesScanners: migrating ==========
-- transaction_open?()
   -> 0.0000s
-- index_exists?(:approval_project_rules, :scanners, {:name=>"tmp_index_approval_project_rules_scanners", :using=>:gin, :where=>"scanners @> '{cluster_image_scanning}'", :algorithm=>:concurrently})
   -> 0.0058s
-- execute("SET statement_timeout TO 0")
   -> 0.0006s
-- add_index(:approval_project_rules, :scanners, {:name=>"tmp_index_approval_project_rules_scanners", :using=>:gin, :where=>"scanners @> '{cluster_image_scanning}'", :algorithm=>:concurrently})
   -> 0.0077s
-- execute("RESET ALL")
   -> 0.0005s
== 20210804202057 AddTmpIndexApprovalProjectRulesScanners: migrated (0.0157s) =

$ SKIP_POST_DEPLOYMENT_MIGRATIONS=true bundle exec rake db:rollback

== 20210804202057 AddTmpIndexApprovalProjectRulesScanners: reverting ==========
-- transaction_open?()
   -> 0.0000s
-- indexes(:approval_project_rules)
   -> 0.0039s
-- execute("SET statement_timeout TO 0")
   -> 0.0005s
-- remove_index(:approval_project_rules, {:algorithm=>:concurrently, :name=>"tmp_index_approval_project_rules_scanners"})
   -> 0.0031s
-- execute("RESET ALL")
   -> 0.0007s
== 20210804202057 AddTmpIndexApprovalProjectRulesScanners: reverted (0.0093s)

$ bundle exec rake db:migrate:up VERSION=20210804150624

== 20210804150624 RemoveClusterImageScanningFromApprovalProjectRules: migrating
-- execute("update approval_project_rules set scanners = array_remove(scanners, 'cluster_image_scanning') where scanners @> '{cluster_image_scanning}'")
   -> 0.0035s
== 20210804150624 RemoveClusterImageScanningFromApprovalProjectRules: migrated (0.0035s)

$ bundle exec rake db:migrate:down VERSION=20210804150624

== 20210804150624 RemoveClusterImageScanningFromApprovalProjectRules: reverting
== 20210804150624 RemoveClusterImageScanningFromApprovalProjectRules: reverted (0.0000s)

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