Supersede `derive_var_bcvacritxfl()` and update vignettes/templates that use it

[manciniedoardo]

Background Information

With the advent of admiral::derive_vars_crit_flag() there is less of a need for admiralophtha::derive_var_bcvacrtixfl(). The latter should be superseded, in line with the {admiral} family's policy to not make many breaking changes post 1.0 releases.

Definition of Done

• [x] derive_var_bcvacritxfl() superseded. See here for an example of a superseded function from {admiral}.
• [x] templates/vignettesalso updated to not use it anymore and use derive_vars_crit_flag(), possibly in conjunction with call_derivation().

[steventing12]

Used call_derivation with derive_vars_crit_flag() to generate CRITy conditions and CRITyFL flags, superseding derive_var_bcvacritxfl()

[manciniedoardo]

Can you please make a pull request with your proposed changes? That way I can review code/changes and propose amendments.