Fix tibble dependency issue in read_sf()

[pdil]

Changes

• Set as_tibble = FALSE in create_us_map(), us_map(), and centroid_labels()

Notes

• This change is due to ggplot2 no longer importing tibble by default (now a Suggests).
  • Move {tibble} to suggests tidyverse/ggplot2#5990
• For the purposes of this package, reading the simple features data as a data frame is suitable.
• Any package consumers that require tibbles can easily convert them using tibble::as_tibble().
• Originally reported in usmap: tibble dependency usmap#117

resolves #53