Recommend to fill gaps in enum values

What does this MR do?

After we merged some of the EE and FOSS enums we ended up with some of the values having gaps between the two groups:

      def self.failure_reasons
        {
          # ...
          data_integrity_failure: 12,
          forward_deployment_failure: 13,
          insufficient_bridge_permissions: 1_001,
          downstream_bridge_project_not_found: 1_002,
          # ...
        }
      end

This MR recommends to fill the gaps (e.g. adding 14) instead of always appending new values to the end of the list (e.g. adding 1_003).

@gitlab-org/maintainers/rails-backend what do you think of this guideline?

Does this MR meet the acceptance criteria?

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