Mark DataContainer.subscript as mutating set

DataContainer already allows mutating via the `append()` method, and all
implementations of DataContainer (Foundation Data, UInt8 array, and
LowEnergyAdvertisingData) support setting via the subscript notation,
but it was not allowed on the generic type because this was marked as
only allowing get access.

Author: dpogueproton

Sources/Bluetooth/Data.swift

@@ -18,7 +18,7 @@ public protocol DataContainer: RandomAccessCollection where Self.Index == Int, S
 
     mutating func reserveCapacity(_ capacity: Int)
 
-    subscript(index: Int) -> UInt8 { get }
+    subscript(index: Int) -> UInt8 { get mutating set }
 
     mutating func append(_ newElement: UInt8)