Make Zlib::GzipWriter.new level and strategy parameters optional

The current signature requires all three positional arguments:

  def initialize: (_Writer io, Integer level, Integer strategy, **untyped opts) -> void

However, the Ruby documentation and actual C implementation show that
both level and strategy have default values (nil), making them optional:

  Zlib::GzipWriter.new(io, level = nil, strategy = nil, options = {})

The documentation examples also demonstrate calling with just io:

  File.open('hoge.gz', 'w') do |f|
    gz = Zlib::GzipWriter.new(f)
    gz.write 'jugemu jugemu gokou no surikire...'
    gz.close
  end

This change updates the signature to allow 1-4 arguments by making
level and strategy optional nilable parameters:

  def initialize: (_Writer io, ?Integer? level, ?Integer? strategy, **untyped opts) -> void

This matches the actual Ruby behavior and resolves false positive type
errors when calling GzipWriter.new with fewer than three arguments.

Author: jwils

stdlib/zlib/0/gzip_writer.rbs

@@ -162,6 +162,6 @@ module Zlib
     # `:external_encoding`, `:internal_encoding` and `:encoding` may be set as in
     # IO::new.
     #
-    def initialize: (_Writer io, ?Integer level, ?Integer strategy, **untyped opts) -> void
+    def initialize: (_Writer io, ?Integer? level, ?Integer? strategy, **untyped opts) -> void
   end
 end