Remove 'UserPortalDataMigrator' steps from the APIM migration document

Description:
The APIM migration document[1] contains the 'Step 3' which is related to the identity components migration of the APIM server.
There we are removing a set of unwanted migration steps when the migration process is based on the APIM servers(Not the WSO2-IS servers as the key-manager).
In that, under the 'Remove the following 3 steps from migration-config.yaml, which are included under version: "5.11.0".' section, we are removing a set of migration steps.
Under the same section, we need to add the below steps as removal steps.

 - name: "UserPortalDataMigrator"
  order: 9
  parameters:
    location: "step4"
    schema: "identity"
- name: "UserPortalDataMigrator"
  order: 10
  parameters:
    location: "step4"
    schema: "um"

Suggested Labels:
migration, migration-to-4.0.0, migration-to-4.1.0, migration-to-4.2.0

Affected Product Version: APIM-4.0.0, APIM-4.1.0, APIM-4.2.0

Related Issues:
https://github.com/wso2-enterprise/wso2-apim-internal/issues/5335

References:
[1] - https://github.com/wso2-enterprise/migration-docs/blob/main/api-manager/migration-docs/apim/apim-4.2.0/apim/upgrading-from-320-to-420.md

Open api-manager/migration-docs/apim/apim-4.2.0/apim/upgrading-from-320-to-420.md and read Step 3, especially the section listing removal steps from migration-config.yaml. Add the two UserPortalDataMigrator entries shown in the issue under that section. Done means the APIM migration document includes both identity and um removal steps with the specified orders and parameters.