Use ByteBuffer#get(int index, byte[] dst) in Loader.java

[eregon]

• This is an absolute bulk get which does not modify the position, and therefore saves from getting, setting and restoring the position.

From #3972 (comment)

[eregon]

The CI passes even though that method doesn't exist on Java 8.
It's because it's using -target 1.8 -source 1.8 and that's not checking if JDK methods exist.
--release=8 (via ext.release = "8") does check it and find it:

javac --release=8 -Xlint -d tmp/java/prism -cp /home/eregon/.rubies/jruby-10.0.4.0/lib/jruby.jar java/org/ruby_lang/prism/AbstractNodeVisitor.java java/org/ruby_lang/prism/Loader.java java/org/ruby_lang/prism/MarkNewlinesVisitor.java java/org/ruby_lang/prism/Nodes.java java/org/ruby_lang/prism/ParseResult.java java/org/ruby_lang/prism/Parser.java java/org/ruby_lang/prism/ParsingOptions.java
warning: [options] source value 8 is obsolete and will be removed in a future release
warning: [options] target value 8 is obsolete and will be removed in a future release
warning: [options] To suppress warnings about obsolete options, use -Xlint:-options.
java/org/ruby_lang/prism/Loader.java:65: error: no suitable method found for get(int,byte[])
                buffer.get(start, bytes);
                      ^
    method ByteBuffer.get() is not applicable
      (actual and formal argument lists differ in length)
    method ByteBuffer.get(int) is not applicable
      (actual and formal argument lists differ in length)
    method ByteBuffer.get(byte[],int,int) is not applicable
      (actual and formal argument lists differ in length)
    method ByteBuffer.get(byte[]) is not applicable
      (actual and formal argument lists differ in length)
1 error
3 warnings

I think this is a good reason to update to JDK 21+ in Prism (which is what JRuby 10 requires).
I think JRuby 9 won't update to a newer Prism from latest master, isn't it? (especially now that Prism has release branches).
cc @headius @enebo