Update annotations GraphQL query props

What does this MR do?

The original proposal for annotations timestamp props were from and to. Because they're generally keywords in most programming languages, its has been renamed to starting_at and ending_at. This will eventually be used in fetching annotations for the monitoring dashboard.

This MR updates the props on the frontend.

Here is the backend piece that shows the annotation attributes https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/api/entities/metrics/dashboard/annotation.rb#L9

module API
  module Entities
    module Metrics
      module Dashboard
        class Annotation < Grape::Entity
          expose :id
          expose :starting_at
          expose :ending_at
          expose :dashboard_path
          expose :description
          expose :environment_id
          expose :cluster_id
        end
      end
    end
  end
end

Note

This feature is behind metrics_dashboard_annotations feature flag.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• 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