Allow feature independent stratifications in $key_join_features

[RasmusSkytte]

Intent

This PR removes a error message that blocks a specific use-case needed for diseasy.

Specifically, if you have stratifications like

rlang::quos(age_group = "0+")

then, diseasystore will currently throw an error since no features are included in the computation.

This PR removes this error

Approach

We now try to perform the grouping at the stratification level and then check if an error is produced.

Also, I had to fully deprecate non-rlang::quos() stratifications.
The documentation has specified to use rlang::quos() for a while but we still allowed it -- that is no longer the case

Known issues

N/A

Checklist

• The PR passes all local unit tests
• I have documented any new features introduced
• If the PR adds a new feature, please add an entry in NEWS.md
• A reviewer is assigned to this PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.50%. Comparing base (cc6c12c) to head (e7f180d).
Report is 13 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #203      +/-   ##
==========================================
+ Coverage   88.44%   88.50%   +0.05%     
==========================================
  Files          18       18              
  Lines        1203     1209       +6     
==========================================
+ Hits         1064     1070       +6     
  Misses        139      139              

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