Create an informative issue (See each section, incomplete templates/issues won't be triaged)

Using the current documentation as a model, please complete the issue template. 

Note: Doc team updates the current version and the two previous versions (n-2). For earlier versions, we will address only high-priority, customer-reported issues for releases in support.

Prerequisite: Start with what we have

Always look at the current documentation to describe the change that is needed. Use the source or portal link for Step 4:

 - Use the Customer Portal: https://access.redhat.com/documentation/en-us/red_hat_advanced_cluster_management_for_kubernetes

 - Use the GitHub link to find the staged docs in the repository: https://github.com/stolostron/rhacm-docs 

Describe the changes in the doc and link to your dev story

Provide info for the following steps:

1. - [ ] Mandatory Add the required version to the Fix version/s field.

2. - [ ] Mandatory Choose the type of documentation change.

      - [ ] New topic in an existing section or new section
      - [X ] Update to an existing topic

3. - [ ] Mandatory for GA content:
            
       - [ ] Add steps and/or other important conceptual information here: 

Add a new section in https://access.redhat.com/documentation/en-us/red_hat_advanced_cluster_management_for_kubernetes/2.8/html-single/add-ons/index#submariner

1.1.9 Upgrading Submariner

Most users don't need to do anything when upgrading Submariner. When ACM is upgraded, Submariner should automatically upgrade to the correct version.

However, for deployments where users used a custom CatalogSource or Subscription Config when installing Submariner Addon on ManagedClusters, they will need to additionaly modify `spec.subscriptionConfig.channel` field in the `SubmarinerConfig` resource for each ManagedCluster.

Channel version to use:

ACM 2.7 - stable-0.14

ACM 2.8 - stable-0.15

            
       - [ ] Add Required access level for the user to complete the task here:
       

       - [ ] Add verification at the end of the task, how does the user verify success (a command to run or a result to see?)
     
     
       - [ ] Add link to dev story here:

4. - [ ] Mandatory for bugs: What is the diff? Clearly define what the problem is, what the change is, and link to the current documentation: