Make taxon_level a char instead of a data var for pool_taxon_counts()

[kelly-sovacool]

Change(s) made

• The taxon_level argument is now a character instead of a data variable. This should prevent hard-to-catch bugs where users give the wrong type. The internal code is actually simpler now with no embracing ({{ taxon_level }}) & only one instance of !!rlang::sym(taxon_level).
• Bump r-lib/actions to v2 in github actions workflows (v1 no longer works).

Checklist

(Strikethrough any points that are not applicable.)

• Write unit tests for any new functionality.
• Update roxygen comments & vignettes if there are any API changes.
• Update NEWS.md if this includes any user-facing changes.
• The check workflow succeeds on your most recent commit.