tidy variable names in ons_population_uk_2023

[tomjemmett]

dataset added in #95, we have a mix of column name styles. I suggest we convert to snake_case (though, I think all of the columns are single words, so just lowercase all column names).

• run janitor::clean_names(ons_population_uk_2023)
• re-run usethis::use_data(ons_population_uk_2023)
• update any documentation that refers to old column names