feature: Support JDK21 (and drop JDK7 support)

[xerial]

This also drops JDK7 support.

An error:

Caused by: java.lang.ClassNotFoundException: java.nio.MemoryBlock
	at java.base/jdk.internal.loader.BuiltinClassLoader.loadClass(BuiltinClassLoader.java:641)
	at java.base/jdk.internal.loader.ClassLoaders$AppClassLoader.loadClass(ClassLoaders.java:188)
	at java.base/java.lang.ClassLoader.loadClass(ClassLoader.java:526)
	at java.base/java.lang.Class.forName0(Native Method)
	at java.base/java.lang.Class.forName(Class.java:421)
	at java.base/java.lang.Class.forName(Class.java:412)
	at org.msgpack.core.buffer.DirectBufferAccess.<clinit>(DirectBufferAccess.java:84)

Related: Remove private DirectByteBuffer(long, int) constructor before JDK 21 GA https://bugs.openjdk.org/browse/JDK-8303083. In JDK21, DirectByteBuffer(long, long) constructor can be used.