Enable compilation with Swift 6 for most targets

[buggmagnet]

This PR enables us to compile most targets with Swift 6.0 except for all the targets that consume NetworkExtensions (duplicated symbols galore and other compilation issues that I want to solve later)

It mostly declares a lot of types as Sendable or disables strict concurrency checks in order to compile.
We will revisit the app piece by piece over time to remove all those @unchecked Sendable since most of the code is already working (as proven by shipping it, and not seeing crashes)

⚠️ We should not merge this PR before we move the CI to Xcode 16 because Xcode 15.0 does not support building with Swift 6.0 ⚠️

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This change is 

[linear]

IOS-741 Fix warnings introduced by xcode 16.

[mojganii]

Reviewed 5 of 294 files at r1, all commit messages.
Reviewable status: 5 of 294 files reviewed, 1 unresolved discussion (waiting on @buggmagnet)

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ios/PacketTunnel/DeviceCheck/DeviceCheckOperation.swift line 137 at r1 (raw file):

        dispatchGroup.enter()
        let accountTask = remoteService.getAccountData(accountNumber: accountNumber) { result in
            accountResult = result

I got this waring which is referring to the concurrency checking:

[acb-mv]

Reviewed 115 of 294 files at r1, all commit messages.
Reviewable status: 115 of 294 files reviewed, 2 unresolved discussions (waiting on @buggmagnet)

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ios/MullvadREST/ApiHandlers/RESTTaskIdentifier.swift line 13 at r1 (raw file):

extension REST {
    private static let nslock = NSLock()
    nonisolated(unsafe) private static var taskCount: UInt32 = 0

Would it make sense to encapsulate this state into an Actor?

[buggmagnet]

Reviewable status: 115 of 294 files reviewed, 2 unresolved discussions (waiting on @acb-mv and @mojganii)

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ios/MullvadREST/ApiHandlers/RESTTaskIdentifier.swift line 13 at r1 (raw file):

Previously, acb-mv wrote…

Would it make sense to encapsulate this state into an Actor?

I believe it would have to be a globally shared actor to achieve the same result, which would then force all network operations to communicate with that actor, which Operation cannot inherently do since they are not actors.

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ios/PacketTunnel/DeviceCheck/DeviceCheckOperation.swift line 137 at r1 (raw file):

Previously, mojganii wrote…

I got this waring which is referring to the concurrency checking:

Yes, this file is used by the PacketTunnelProvider which is still using Swift 5.0 compilation mode, hence the warnings.
However we can silence those warnings.