Add upcoming feature flag to SE-0337 to match inclusion in compiler (#2384)

- Add 'Implemented in 6.0' to match guidance the flag should not be
  used as an upcoming feature flag until Swift 6.0
- Annotate that flag is always enabled in Swift 6 language mode

Author: dempseyatgithub

proposals/0337-support-incremental-migration-to-concurrency-checking.md

@@ -4,6 +4,7 @@
 * Authors: [Doug Gregor](https://github.com/DougGregor), [Becca Royal-Gordon](https://github.com/beccadax)
 * Review Manager: [Ben Cohen](https://github.com/AirspeedSwift)
 * Status: **Implemented (Swift 5.6)**
+* Upcoming Feature Flag: `StrictConcurrency` (Implemented in Swift 6.0) (Enabled in Swift 6 language mode)
 * Implementation: [Pull request](https://github.com/apple/swift/pull/40680), [Linux toolchain](https://ci.swift.org/job/swift-PR-toolchain-Linux/761//artifact/branch-main/swift-PR-40680-761-ubuntu16.04.tar.gz), [macOS toolchain](https://ci.swift.org/job/swift-PR-toolchain-osx/1256//artifact/branch-main/swift-PR-40680-1256-osx.tar.gz)
 
 ## Introduction