compatibility with tidyselect 1.1

`tidyselect` 1.1 brings a new `any_of()` that we could use to “forgivingly” drop variables that might or might not be present:

```
foo %>% select(-any_of(...))
```

We should also consider whether the new `select(., where(...))` syntax might be useful in our `vartools`
package (or in any of our other in-house packages or scripts).

Review the vartools package and any in-house packages or scripts that use variable-dropping or selection helpers; start by identifying those call sites and their tidyselect version constraints. Evaluate whether tidyselect 1.1's any_of() and select(., where(...)) apply, then implement the agreed compatibility changes and verify the affected selection behavior.

Written by the indexing model from the issue text.