Investigate migration to Swift 6

[sosnovsky]

Xcode 16 includes Swift 6, which has some changes requiring migration of existing codebase - https://www.swift.org/migration/documentation/migrationguide/

Let's check if switching to Swift 6 will require too many changes for our existing code, or can be done quite quickly.

[sosnovsky]

Yeah, it seems quite complicated, so no need to migrate for now, I removed actionable label and moved it to the next milestone.