Add upcoming feature flag to SE-0352 to match inclusion in compiler (#2381)

* Add upcoming feature flag to SE-0352 to match inclusion in compiler

- Use 'Implemented in' since flags were not added until Swift 5.8
- Annotate that flag is always enabled in Swift 6 language mode

* Make suggested change to reflect when upcoming feature flag was implemented.

Co-authored-by: Doug Gregor <[email protected]>

---------

Co-authored-by: Doug Gregor <[email protected]>

Author: dempseyatgithub

proposals/0352-implicit-open-existentials.md

@@ -4,6 +4,7 @@
 * Authors: [Doug Gregor](https://github.com/DougGregor)
 * Review Manager: [Joe Groff](https://github.com/jckarter)
 * Status: **Implemented (Swift 5.7)**
+* Upcoming Feature Flag: `ImplicitOpenExistentials` (Implemented in Swift 6.0) (Enabled in Swift 6 language mode)
 * Implementation: [apple/swift#41996](https://github.com/apple/swift/pull/41996), [macOS toolchain](https://ci.swift.org/job/swift-PR-toolchain-macos/120/artifact/branch-main/swift-PR-41996-120-osx.tar.gz)
 * Decision Notes: [Acceptance](https://forums.swift.org/t/accepted-se-0352-implicitly-opened-existentials/57553)
 * Previous Revision: [1](https://github.com/apple/swift-evolution/blob/77374319a7d70c866bd197faada46ecfce461645/proposals/0352-implicit-open-existentials.md)