docs: Use AWS SDK v3 in example

[j15e]

This PR updates the AWS SDK documentation example to use the SDK v3 as discussed in #1470

It pretty much now implement the suggested example in the README itself:

opentelemetry-ruby-contrib/instrumentation/aws_sdk/README.md

Lines 47 to 58 in 51f6328

	Using later versions of these gems will give more details on the internal spans.
	See below for example usage:
	```ruby
	# configures the OpenTelemetry SDK with instrumentation defaults
	OpenTelemetry::SDK.configure do |c|
	c.use 'OpenTelemetry::Instrumentation::AwsSdk'
	end
	# create open-telemetry provider and pass to client config
	otel_provider = Aws::Telemetry::OTelProvider.new
	client = Aws::S3::Client.new(telemetry_provider: otel_provider)
	```

And I added the version restriction in the example Gemfile to avoid any confusion on the SDK version used.

I also added rexml, because starting from Ruby 3.0 it isn't shipped with Ruby core, but aws-sdk-core needs an XML parser. It works on the AWS SDK CI without specifying rexml, because webmock requires rexml, but we don't have this gem in the example.

If you run for example AWS_ACCESS_KEY_ID=123 AWS_SECRET_ACCESS_KEY=qwer AWS_REGION=us-east-1 ruby trace_demonstration.rb you will now get the example trace working :

[kaylareopelle]

cc @jterapin @alextwoods

[jterapin]

This is great! Thank you for the contribution! Much appreciated.