use lifecycle package to manage deprecation / stability / experimental

[njtierney]

Perhaps use https://lifecycle.r-lib.org/articles/stages.html#deprecated for this deprecation:

e.g,

greta/R/optimisers.R

Line 41 in a07d84f

optimiser_deprecation_warning <- function() {

And other functions to signal which functions are experimental / superceded / stable etc.

Perhaps a lower priority but worth discussing I think.