replace as_tibble with new_tibble in ungroup()

[mkoohafkan]

Make ungroup() handle attributes in a way that is consistent with other dplyr functions by using new_tibble() instead of as_tibble().

This is also future-proof if it is later decided that new_tibble() should drop attributes.

Fixes #6774 without affecting discussion in tidyverse/tibble#769

[krlmlr]

Thanks, this looks good to me. It seems that other attributes are preserved here:

tibble::new_tibble(
  structure(list(a = 1), foo = "bar", let_it = "go"),
  let_it = NULL
) |>
  attributes()
#> $class
#> [1] "tbl_df"     "tbl"        "data.frame"
#> 
#> $row.names
#> [1] 1
#> 
#> $names
#> [1] "a"
#> 
#> $foo
#> [1] "bar"

^{Created on 2023-08-23 with reprex v2.0.2}