Clean up min data rate implementation

[sbordet]

Jetty version(s)
13.0.x

Jetty Environment
eeN

Description
In Jetty 11 we had support for min read/write data rate, baked into HttpOutput and configured via HttpConfiguration.

In Jetty 12+, this mechanism has been replaced by StatisticsHandler.MinimumDataRateHandler.

The clean up should:

• Remove the leftover implementation from HttpOutput (e.g. HttpOutput._firstByteNanoTime) and AsyncContentProducer.
• Introduce a Jetty module that configures StatisticsHandler.MinimumDataRateHandler from the HttpConfiguration parameters.

[sbordet]

After resolution of #15267, what remains is:

• Remove leftover from AsyncContentProducer.
• Remove deprecated methods for HttpConfiguration.min[Request|Response]DataRate.
• Update tests that were using HttpConfiguration.min[Request|Response]DataRate methods.
• Update the migration guide to mention that now min data rate enforcement is opt-in.