use feature-specific guard for @retroactive (#2581)

Motivation:

We should use the more granular guard for uses of @retroactive rather
than coarse swift versions to guard against corner-cases

Modifications:

Switch `#if compiler(>=5.11)` for `#if hasFeature(RetroactiveAttribute)`

Result:

No change in most cases, more protected against corner-cases.

Sources/NIOFoundationCompat/ByteBuffer-foundation.swift

@@ -375,7 +375,8 @@ extension ByteBufferAllocator {
 }
 
 // MARK: - Conformances
-#if compiler(>=5.11)
+#if swift(>=5.8)
+#if hasFeature(RetroactiveAttribute)
 extension ByteBufferView: @retroactive ContiguousBytes {}
 extension ByteBufferView: @retroactive DataProtocol {}
 extension ByteBufferView: @retroactive MutableDataProtocol {}
@@ -384,6 +385,11 @@ extension ByteBufferView: ContiguousBytes {}
 extension ByteBufferView: DataProtocol {}
 extension ByteBufferView: MutableDataProtocol {}
 #endif
+#else
+extension ByteBufferView: ContiguousBytes {}
+extension ByteBufferView: DataProtocol {}
+extension ByteBufferView: MutableDataProtocol {}
+#endif
 
 extension ByteBufferView {
     public typealias Regions = CollectionOfOne<ByteBufferView>