Remove shebang from example files

Since the example files are not executable binaries, this PR removes the shebangs.

This ensures that all examples are consistently executed via `ruby` command.
It also removes the executable permission from `examples/stdio_server.rb` using `chmod 444`,
bringing it in line with the other example files.

examples/streamable_http_client.rb

@@ -1,4 +1,3 @@
-#!/usr/bin/env ruby
 # frozen_string_literal: true
 
 require "net/http"