Add support for upcoming logger 1.4.3

logger 1.4.3 will have ruby/logger#85. It initializes a new instance
variable in `Logger#initialize`. `Jekyll::Stevenson < ::Logger`
doesn't use `super`. So the new instance variable isn't initialized in
`Jekyll::Stevenson`. And it causes an exception:

```text
/tmp/local/lib/ruby/3.3.0+0/logger.rb:385:in `level': undefined method `[]' for nil (NoMethodError)

    @level_override[Fiber.current] || @Level
                   ^^^^^^^^^^^^^^^
	from /tmp/local/lib/ruby/gems/3.3.0+0/gems/jekyll-4.3.2/lib/jekyll/log_adapter.rb:45:in `adjust_verbosity'
	from /tmp/local/lib/ruby/gems/3.3.0+0/gems/jekyll-4.3.2/lib/jekyll/configuration.rb:143:in `config_files'
	from /tmp/local/lib/ruby/gems/3.3.0+0/gems/jekyll-4.3.2/lib/jekyll.rb:118:in `configuration'
	from /tmp/local/lib/ruby/gems/3.3.0+0/gems/jekyll-4.3.2/lib/jekyll/command.rb:44:in `configuration_from_options'
	from /tmp/local/lib/ruby/gems/3.3.0+0/gems/jekyll-4.3.2/lib/jekyll/commands/serve.rb:83:in `block (2 levels) in init_with_program'
	from /tmp/local/lib/ruby/gems/3.3.0+0/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `block in execute'
	from /tmp/local/lib/ruby/gems/3.3.0+0/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `each'
	from /tmp/local/lib/ruby/gems/3.3.0+0/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `execute'
	from /tmp/local/lib/ruby/gems/3.3.0+0/gems/mercenary-0.4.0/lib/mercenary/program.rb:44:in `go'
	from /tmp/local/lib/ruby/gems/3.3.0+0/gems/mercenary-0.4.0/lib/mercenary.rb:21:in `program'
	from /tmp/local/lib/ruby/gems/3.3.0+0/gems/jekyll-4.3.2/exe/jekyll:15:in `<top (required)>'
	from /tmp/local/bin/jekyll:25:in `load'
	from /tmp/local/bin/jekyll:25:in `<top (required)>'
	from /tmp/local/lib/ruby/3.3.0+0/bundler/cli/exec.rb:58:in `load'
	from /tmp/local/lib/ruby/3.3.0+0/bundler/cli/exec.rb:58:in `kernel_load'
	from /tmp/local/lib/ruby/3.3.0+0/bundler/cli/exec.rb:23:in `run'
	from /tmp/local/lib/ruby/3.3.0+0/bundler/cli.rb:492:in `exec'
	from /tmp/local/lib/ruby/3.3.0+0/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
	from /tmp/local/lib/ruby/3.3.0+0/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
	from /tmp/local/lib/ruby/3.3.0+0/bundler/vendor/thor/lib/thor.rb:392:in `dispatch'
	from /tmp/local/lib/ruby/3.3.0+0/bundler/cli.rb:34:in `dispatch'
	from /tmp/local/lib/ruby/3.3.0+0/bundler/vendor/thor/lib/thor/base.rb:485:in `start'
	from /tmp/local/lib/ruby/3.3.0+0/bundler/cli.rb:28:in `start'
	from /tmp/local/lib/ruby/gems/3.3.0+0/gems/bundler-2.5.0.dev/libexec/bundle:37:in `block in <top (required)>'
	from /tmp/local/lib/ruby/3.3.0+0/bundler/friendly_errors.rb:117:in `with_friendly_errors'
	from /tmp/local/lib/ruby/gems/3.3.0+0/gems/bundler-2.5.0.dev/libexec/bundle:29:in `<top (required)>'
	from /tmp/local/bin/bundle:25:in `load'
	from /tmp/local/bin/bundle:25:in `<main>'
```

How about using `super` instead of implementing our
`Jekyll::Stevenson#initialize` to reduce maintenance cost?

lib/jekyll/stevenson.rb

@@ -3,13 +3,10 @@
 module Jekyll
   class Stevenson < ::Logger
     def initialize
-      @progname = nil
-      @level = DEBUG
-      @default_formatter = Formatter.new
-      @logdev = $stdout
-      @formatter = proc do |_, _, _, msg|
+      formatter = proc do |_, _, _, msg|
         msg.to_s
       end
+      super($stdout, formatter: formatter)
     end
 
     def add(severity, message = nil, progname = nil)