Escalate coef(directional_information) deprecation and flip default to TRUE

[joethorley]

#71 added the directional_information argument to the coef() methods with a deprecate_soft("0.7.0", ...) signal that the default will change from FALSE to TRUE in a future release (warn_default_directional_information() in R/coef.R).

Per the escalation policy, about one year after 0.7.0 ships to CRAN:

1. Escalate the signal from deprecate_soft() to deprecate_warn() so indirect users are warned.
2. After a further transition period, flip the default to TRUE and remove the warning.

The existing lifecycle::expect_deprecated() tests in test-coef.R cover the signal; update them at each stage.