Change back-end to use {openxlsx2}

[matt-dray]

{openxlsx2} will replace {openxlsx} at some point, presumably? Investigate its potential, but don't integrate until CRAN release and some stability.

[matt-dray]

{openxlsx2} has continued to develop and is on CRAN, but it's still not a v1.

There's a vignette to explain some of the changes.

Consider forking {a11ytables} and creating {a11ytables2} (or complete rename) using {openxlsx2}.

[matt-dray]

A nice accessibility feature that appeared in a test of the package is that automatic cell colour is supported through wb_color(auto = "1"). Demo in a Gist.