Rename RubyVM::MJIT to RubyVM::JIT

because the name "MJIT" is an internal code name, it's inconsistent with
--jit while they are related to each other, and I want to discourage future
JIT implementation-specific (e.g. MJIT-specific) APIs by this rename.

[Feature #17490]

test/reline/test_within_pipe.rb

@@ -8,7 +8,7 @@ def setup
     @reader, @output_writer = IO.pipe((RELINE_TEST_ENCODING rescue Encoding.default_external))
     @output = Reline.output = @output_writer
     @config = Reline.send(:core).config
-    @config.keyseq_timeout *= 600 if defined?(RubyVM::MJIT) && RubyVM::MJIT.enabled? # for --jit-wait CI
+    @config.keyseq_timeout *= 600 if defined?(RubyVM::JIT) && RubyVM::JIT.enabled? # for --jit-wait CI
     @line_editor = Reline.send(:core).line_editor
   end