[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

Tests/LCEssentialsTests/APIUploadTests.swift

@@ -87,7 +87,6 @@ final class APIUploadTests: XCTestCase {
         XCTAssertEqual(tempCountInDir(dir), before)
     }
 
-    @available(iOS 15.0, *)
     func testProgressOverloadDeliversFinalCompletionAndDecodes() async throws {
         StubURLProtocol.setStub(.init(statusCode: 200, body: Data(#"{"ok":true}"#.utf8)))