Consider simplifying external vector warning

[DavisVaughan]

See tidyverse/dplyr#6718

CC @hadley, who provided the suggested simplifications

The existing warning seems to confuse some people since it mentions select() but that isn't always the verb being used

# Using an external vector in selections was deprecated in tidyselect 1.1.0.
# ℹ Please use `all_of()` or `any_of()` instead.
#   # Was:
#   data %>% select(var)
#   
#   # Now:
#   data %>% select(all_of(var))

What if we just simplified this one to

# Using an external vector in selections was deprecated in tidyselect 1.1.0.
# ℹ Please use `all_of(var)` or `any_of(var)` instead.

Or

# Using an external vector in selections was deprecated in tidyselect 1.1.0.
# ℹ Use `all_of(var)` to error if any elements of `var` are not found.
# ℹ Use `any_of(var)` to silently drop elements of `var` that are not found.