Deprecate named kwarg in 3.x branch

[palday]

cf. #469

Closes #499

[codecov]

Codecov Report

Merging #507 (2c28fcf) into v3.x (efe8ec2) will decrease coverage by 0.04%.
The diff coverage is 66.66%.

❗ Current head 2c28fcf differs from pull request most recent head 88e2e3e. Consider uploading reports for the commit 88e2e3e to get more accurate results

@@            Coverage Diff             @@
##             v3.x     #507      +/-   ##
==========================================
- Coverage   93.96%   93.91%   -0.05%     
==========================================
  Files          24       24              
  Lines        2103     2105       +2     
==========================================
+ Hits         1976     1977       +1     
- Misses        127      128       +1     

Impacted Files	Coverage Δ
src/linearmixedmodel.jl	94.56% <66.66%> (-0.23%)	⬇️

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[palday]

@ararslan This came back to haunt us, so I took your suggestion for a backport.