Update 'Upcoming feature flag' name

Change the header field 'Feature Identifier' to
'Upcoming feature flag' to be consistent with the
use of that name in the existing proposals with
upcoming feature flags as added in 70f9605.

Author: dempseyatgithub

proposal-templates/0000-swift-template.md

@@ -8,7 +8,7 @@
 * Roadmap: *if applicable* [Roadmap Name](https://forums.swift.org/...))
 * Bug: *if applicable* [apple/swift#NNNNN](https://github.com/apple/swift/issues/NNNNN)
 * Implementation: [apple/swift#NNNNN](https://github.com/apple/swift/pull/NNNNN) or [apple/swift-evolution-staging#NNNNN](https://github.com/apple/swift-evolution-staging/pull/NNNNN)
-* Feature Identifier: *if applicable* `MyFeatureName`
+* Upcoming feature flag: *if applicable* `MyFeatureName`
 * Previous Proposal: *if applicable* [SE-XXXX](XXXX-filename.md)
 * Previous Revision: *if applicable* [1](https://github.com/apple/swift-evolution/blob/...commit-ID.../proposals/NNNN-filename.md)
 * Review: ([pitch](https://forums.swift.org/...))
@@ -62,9 +62,9 @@ been committed to the main branch (as an experimental feature), say
 that and specify the experimental feature flag.  If the implementation
 is spread across multiple PRs, just link to the most important ones.
 
-`Feature Identifier` should be the feature name used to identify this
+`Upcoming feature flag` should be the feature name used to identify this
 feature under [SE-0362](https://github.com/apple/swift-evolution/blob/main/proposals/0362-piecemeal-future-features.md#proposals-define-their-own-feature-identifier).
-Not all proposals need a feature identifier.  You should think about
+Not all proposals need an upcoming feature flag.  You should think about
 whether one would be useful for your proposal as part of filling this
 field out.