Make 'Upcoming Feature Flag' consistent with other proposal heading names

- All other proposal heading fields use title case, update 'Upcoming feature flag' to match
- Update proposal template and existing proposals using this field

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)
-* Upcoming feature flag: *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/...))
@@ -62,7 +62,7 @@ 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.
 
-`Upcoming feature flag` 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 an upcoming feature flag.  You should think about
 whether one would be useful for your proposal as part of filling this

proposals/0274-magic-file.md

@@ -4,7 +4,7 @@
 * Authors: [Becca Royal-Gordon](https://github.com/beccadax), [Dave DeLong](https://github.com/davedelong)
 * Review Manager: [Ben Cohen](https://github.com/airspeedswift/)
 * Status: **Implemented (Swift 5.8)**
-* Upcoming feature flag: `ConciseMagicFile`
+* Upcoming Feature Flag: `ConciseMagicFile`
 * Decision Notes: [Review #1](https://forums.swift.org/t/se-0274-concise-magic-file-names/32373/50), [Review #2](https://forums.swift.org/t/re-review-se-0274-concise-magic-file-names/33171/11), [Additional Commentary](https://forums.swift.org/t/revisiting-the-source-compatibility-impact-of-se-0274-concise-magic-file-names/37720)
 * Next Proposal: [SE-0285](0285-ease-pound-file-transition.md)
 

proposals/0286-forward-scan-trailing-closures.md

@@ -4,7 +4,7 @@
 * Author: [Doug Gregor](https://github.com/DougGregor)
 * Review Manager: [John McCall](https://github.com/rjmccall)
 * Status: **Implemented (Swift 5.3)**
-* Upcoming feature flag: `ForwardTrailingClosures` (implemented in Swift 5.8)
+* Upcoming Feature Flag: `ForwardTrailingClosures` (implemented in Swift 5.8)
 * Implementation: [apple/swift#33092](https://github.com/apple/swift/pull/33092)
 * Toolchains: [Linux](https://ci.swift.org/job/swift-PR-toolchain-Linux/404//artifact/branch-master/swift-PR-33092-404-ubuntu16.04.tar.gz), [macOS](https://ci.swift.org/job/swift-PR-toolchain-osx/579//artifact/branch-master/swift-PR-33092-579-osx.tar.gz)
 * Discussion: ([Pitch #1](https://forums.swift.org/t/pitch-1-forward-scan-matching-for-trailing-closures-source-breaking/38162)), ([Pitch #2](https://forums.swift.org/t/pitch-2-forward-scan-matching-for-trailing-closures/38491))

proposals/0335-existential-any.md

@@ -4,7 +4,7 @@
 * Authors: [Holly Borla](https://github.com/hborla)
 * Review Manager: [Doug Gregor](https://github.com/DougGregor)
 * Status: **Implemented (Swift 5.6)**
-* Upcoming feature flag: `ExistentialAny` (implemented in Swift 5.8)
+* Upcoming Feature Flag: `ExistentialAny` (implemented in Swift 5.8)
 * Implementation: [apple/swift#40282](https://github.com/apple/swift/pull/40282)
 * Decision Notes: [Acceptance](https://forums.swift.org/t/accepted-with-modifications-se-0335-introduce-existential-any/54504)
 

proposals/0354-regex-literals.md

@@ -4,7 +4,7 @@
 * Authors: [Hamish Knight](https://github.com/hamishknight), [Michael Ilseman](https://github.com/milseman), [David Ewing](https://github.com/DaveEwing)
 * Review Manager: [Ben Cohen](https://github.com/airspeedswift)
 * Status: **Implemented (Swift 5.7)**
-* Upcoming feature flag: `BareSlashRegexLiterals` (implemented in Swift 5.8)
+* Upcoming Feature Flag: `BareSlashRegexLiterals` (implemented in Swift 5.8)
 * Implementation: [apple/swift#42119](https://github.com/apple/swift/pull/42119), [apple/swift#58835](https://github.com/apple/swift/pull/58835)
   * Bare slash syntax `/.../` available with `-enable-bare-slash-regex`
 * Review: ([first pitch](https://forums.swift.org/t/pitch-regular-expression-literals/52820))