Transfer ownership of `terraform-azurerm-avm-ptn-policyassignment`

After speaking with @bjornhofer he would like to transfer the ownership of `terraform-azurerm-avm-ptn-policyassignment` to just himself and remove @steph409 from the ownership of the module.

Please can the @Azure/avm-core-team-technical-terraform process this change and advise @bjornhofer & @steph409 of any steps they need to take.

> This process may also need documenting on the AVM site for future ease 👍

No repository file, test, or entry point is named. Start by confirming the AVM core team's ownership-transfer process and the steps required from @bjornhofer and @steph409; done means the module ownership reflects the request and both people are advised. If the process is not already documented, capture the agreed steps on the AVM site.

Written by the indexing model from the issue text.