[api-upload-refactor] Set deployment target to iOS 15, simplify upload

- Package.swift: .iOS(.v13) -> .iOS(.v15), .watchOS(.v6) -> .v8
  (README already stated iOS 15; watchOS 8 needed for URLSession.upload(for:fromFile:))
- drop the iOS-13 uploadTask continuation bridge; both upload overloads now
  share one runUpload using session.upload(for:fromFile:delegate:)
- remove now-redundant @available(iOS 15) from the progress overload and test
- Documentation/API.md: drop iOS-version caveats

Package.resolved

@@ -1,5 +1,5 @@
 {
-  "originHash" : "50e672f8971a68675dcb3ca27b4bd4832c7e96f2216ae062f002554d445c6714",
+  "originHash" : "6e3fcf8724d6b7d2d0bbffc25b4ec3b9d8b65a6ef53d157498d5447fd26cf5db",
   "pins" : [
     {
       "identity" : "lcecryptokitbinary",