refactor: consistent default for trainer arg

[dshemetov]

Checklist

Please:

• [x] Make sure this PR is against "dev", not "main".
• [x] Request a review from one of the current epipredict main reviewers: dajmcdon.
• [x] Make sure to bump the version number in DESCRIPTION and NEWS.md. Always increment the patch version number (the third number), unless you are making a release PR from dev to main, in which case increment the minor version number (the second number).
• [x] Describe changes made in NEWS.md, making sure breaking changes (backwards-incompatible changes to the documented interface) are noted. Collect the changes under the next release number (e.g. if you are on 0.7.2, then write your changes under the 0.8 heading).
• [x] Consider pinning the epiprocess version in the DESCRIPTION file if
  • You anticipate breaking changes in epiprocess soon
  • You want to co-develop features in epipredict and epiprocess

Change explanations for reviewer

arx_fcast_epi_workflow() and arx_class_epi_workflow() now default to trainer = parsnip::logistic_reg() to match their more canned versions.

Magic GitHub syntax to mark associated Issue(s) as resolved when this is merged into the default branch

[dshemetov]

Allowed trainer = NULL again 👍