Update tools version to 5.9 (#508)

Motivation:

When we dropped 5.8 support we didn't raise the tools version. The
result is that 5.8 compilers still resolve newer versions, including
2.29.1. Unfortunately, 2.29.1 doesn't compile.

Modifications:

Lift the tools version to 5.9

Result:

Users won't accidentally resolve this from 5.8.

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:5.8
+// swift-tools-version:5.9
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the SwiftNIO open source project