warn when parallel processing with foreach

[simonpcouch]

Related to #866, a revert-ish of e79aac1.

With this PR:

library(tidymodels)

library(doMC)
registerDoMC(10)

res <- fit_resamples(linear_reg(), mpg ~ ., bootstraps(mtcars))
#> Warning: 
#>  ! tune detected a parallel backend registered with foreach but no backend
#>   registered with future.
#> ℹ Support for parallel processing with foreach was soft-deprecated in tune
#>   1.2.1.
#> ℹ See ?parallelism (`?tune::parallelism()`) to learn more.

^{Created on 2024-03-22 with reprex v2.1.0}

[simonpcouch]

This URL will 404 until merged. :)

[EmilHvitfeldt]

🚀

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