Remove `survivalmodels` dependency

[bblodfon]

Since RaphaelS1/survivalmodels has been archived (July 2024), we’re effectively left with only LearnerSurvAkritas and LearnerSurvParametric.

My plan is to:

• move LearnerSurvAkritas into a small standalone package (code is already faster, just needs packaging/CRAN)
• handle parametric survival either by adding a survival::survreg() learner to mlr3survival, or by relying on flexsurv::flexsurvreg(), which we already have