Update `predict(type = "conf_int")` for stan engine to use `posterior_epred()`

[hfrick]

Looks like posterior_linpred() has been deprecated in favor of posterior_epred()

library(poissonreg)
#> Loading required package: parsnip

res_xy <- fit_xy(
  poisson_reg() %>%
    set_engine("stan", seed = 1333, chains = 10, iter = 1000, refresh = 0),
  x = seniors[, 1:3],
  y = seniors$count
)

confidence_parsnip <- 
  predict(res_xy,
          new_data = seniors[1:5,],
          type = "conf_int",
          level = 0.93)
#> Instead of posterior_linpred(..., transform=TRUE) please call posterior_epred(), which provides equivalent functionality.

^{Created on 2023-11-10 with reprex v2.0.2}