Rename report type Running Container Scanning to Cluster Image Scanning

What does this MR do?

In !64220 (comment 611139342), during the documentation review we've decided to rename the feature to Cluster Image Scanning from Running Container Scanning to avoid confusion with Container Scanning scans and to better describe what this feature is offering. This MR reflects that.

It simply renames the names mentioned in database and code to match updated name (previous name was introduced in current release %14.1).

Migrations

⋊> ~/D/g/gitlab on 330713-rename-running-container-scanning-to-cluster-image-scanning ⨯ env VERBOSE=true bundle exec rake db:migrate:up VERSION=20210627204936
== 20210627204936 AddPlanLimitsMaxSizeClusterImageScanningColumn: migrating ===
-- add_column(:plan_limits, :ci_max_artifact_size_cluster_image_scanning, :integer, {:null=>false, :default=>0})
   -> 0.0045s
== 20210627204936 AddPlanLimitsMaxSizeClusterImageScanningColumn: migrated (0.0046s) 

⋊> ~/D/g/gitlab on 330713-rename-running-container-scanning-to-cluster-image-scanning ⨯ env VERBOSE=true bundle exec rake db:migrate:down VERSION=20210627204936               13:04:34
== 20210627204936 AddPlanLimitsMaxSizeClusterImageScanningColumn: reverting ===
-- remove_column(:plan_limits, :ci_max_artifact_size_cluster_image_scanning)
   -> 0.0022s
== 20210627204936 AddPlanLimitsMaxSizeClusterImageScanningColumn: reverted (0.0023s) 

Does this MR meet the acceptance criteria?

Conformity

• I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• [-] I have tested this MR in all supported browsers, or it's not needed.
• [-] I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.

Security

Does this MR contain changes to processing or storing of credentials or tokens, authorization and authentication methods or other items described in the security review guidelines? If not, then delete this Security section.

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Related to #330713 (closed)