Remove obsolete addorset from the API

[kutal10]

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Fixes https://github.com/akkadotnet/akka.net/issues/7463

Changes

Eliminates the unneeded AddOrSet method API from 1.6 - AddOrReturn is also modified to account for this.

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• [x] This change follows the Akka.NET API Compatibility Guidelines.
• [x] This change follows the Akka.NET Wire Compatibility Guidelines.
• [x] I have reviewed my own pull request.

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[Aaronontheweb]

Thanks @kutal10 ! You might need to run the API approvals to get this to work: https://getakka.net/community/contributing/api-changes-compatibility.html

[kutal10]

Thanks @kutal10 ! You might need to run the API approvals to get this to work: https://getakka.net/community/contributing/api-changes-compatibility.html

@Aaronontheweb Yep followed the guide - PR now has the API txt files updated!