Default to JDK latest in jt

* Since standalones in releases use only JDK latest it seems best to use it by default.
* Makes it easier to test features like Panama which is stable in JDK 22+.

Author: eregon

tool/jt.rb

@@ -67,7 +67,7 @@
 ENV['GEM_HOME'] = File.expand_path(ENV['GEM_HOME']) if ENV['GEM_HOME']
 
 JDK_VERSIONS = %w[latest 21]
-DEFAULT_JDK_VERSION = JDK_VERSIONS.last
+DEFAULT_JDK_VERSION = 'latest'
 
 MRI_TEST_RELATIVE_PREFIX = 'test/mri/tests'
 MRI_TEST_PREFIX = "#{TRUFFLERUBY_DIR}/#{MRI_TEST_RELATIVE_PREFIX}"