Set rake-compiler source and target to Java 8 (#1071)

This patch forces rake-compiler to emit bytecode for Java 8 level
of JVM. rake-compiler currently defaults to Java 1.7 source and
target, but that version is no longer supported by recent JDKs
such as Java 21 (see rake-compiler/rake-compiler#242
for a patch to update rake-compiler).

This patch sets our minimum Java level to 8, bypassing the default
in rake-compiler and allowing builds on Java 21+.

Rakefile

@@ -12,6 +12,8 @@ ENV['JRUBY_HOME'] = ENV['CONCURRENT_JRUBY_HOME'] if ENV['CONCURRENT_JRUBY_HOME']
 Rake::JavaExtensionTask.new('concurrent_ruby', core_gemspec) do |ext|
   ext.ext_dir = 'ext/concurrent-ruby'
   ext.lib_dir = 'lib/concurrent-ruby/concurrent'
+  ext.source_version = '8'
+  ext.target_version = '8'
 end
 
 if RUBY_ENGINE == 'ruby'