Update retrofit from 2.9.0 to 2.11.0

[SimonMarquis]

• Replace Jake Wharton's Kotlinx Serialization converter with new retrofit's converter-kotlinx-serialization.
• Remove custom Proguard rules that are now part of the dependency: https://github.com/square/retrofit/tree/trunk/retrofit-response-type-keeper#readme

Changelog: https://github.com/square/retrofit/blob/trunk/CHANGELOG.md

This is an updated version of the initial update PR that remained untouched for too long:

• #1344