Update stansummary to use new rhat diagnostic

[mitzimorris]

Summary:

Stan PR stan-dev/stan#3266 update Rhat statistic.
Change stansummary accordingly

Description:

Open questions: change computation of ESS as well to match R-ecosystem bulk / tail estimates?

Adding R_hat_bulk and R_hat_tail; no ESS_bulk/ESS_tale (as of yet)

Current Output:

The current output. Knowing what is the current behavior is useful.

Expected Output:

Describe what you expect the output to be. Knowing the correct behavior is also very useful.

Additional Information:

Provide any additional information here.

Current Version:

v2.34.1

[mitzimorris]

answering question:

Open questions: change computation of ESS as well to match R-ecosystem bulk / tail estimates?

stan-dev/stan#3266 only recalculated R-hat; not computing bulk/tail for ESS.