Remove the JNA dependency by using JDK Unix domain socket support

Since CONJ-1354 replaced waffle-jna with the JDK native GSS-API bridge, JNA (jna and jna-platform, both optional) is only used by the socket layer:

• UnixDomainSocket binds libc socket/connect/recv/send/close through Native.register("c") to implement the localSocket option.
• NamedPipeSocket calls Kernel32.WaitNamedPipe when a pipe is busy, with an existing sleep-and-retry fallback used when JNA is absent.
• SocketUtility calls Platform.getOSType() only to detect JNA presence.

Java 16 provides native Unix domain socket support through SocketChannel.open(StandardProtocolFamily.UNIX) and UnixDomainSocketAddress, which removes the need for a native access library.

goal is to:

• Add a java16 multi-release source tree with a UnixDomainSocket built on a UNIX SocketChannel, and the matching compile-java-16 execution in the pom profiles.
• Make the Java 8 baseline UnixDomainSocket fail with a clear "requires Java 16+" error.
• Drop the WaitNamedPipe branch from NamedPipeSocket and the JNA presence check from SocketUtility.
• Remove JNA from the pom profiles, module-info.java (requires static com.sun.jna*), bnd.bnd Import-Package, and the SBOM vendor list.

(OpenJDK java 8 EOL is end of November 2026)