Migrate to Swift 6

[anferbui]

Feature Name

Swift 6 Migration

Description

We should enable the Swift 6 language mode for Swift-DocC, and resolve any failing compiler safety checks.

We can do so incrementally by:

1. Enabling complete concurrency checking
2. Enabling the Swift 6 language mode

Swift 6 is enabled per-target, so there are multiple targets we potentially need to migrate:

• SwiftDocC
• SwiftDocCUtilities
• SwiftDocCTestUtilities?

Motivation

With the Swift 6 language mode, the compiler can now guarantee that concurrent programs are free of data races:
https://www.swift.org/migration/documentation/migrationguide/

Importance

As Swift-DocC uses concurrency in some areas of its code, it is important to ensure that these areas are data-race-free.

Alternatives Considered

No response