compat fixes for Julia 1.7

[palday]

Closes #532

• deleted a dead path now that we've dropped pre 1.6 support
• added an internal wrapper method to dodge some deprecation warnings in 1.7+

[codecov]

Codecov Report

Merging #547 (0afce6c) into main (26a87d5) will increase coverage by 1.71%.
The diff coverage is 97.05%.

@@            Coverage Diff             @@
##             main     #547      +/-   ##
==========================================
+ Coverage   94.44%   96.16%   +1.71%     
==========================================
  Files          27       27              
  Lines        2468     2425      -43     
==========================================
+ Hits         2331     2332       +1     
+ Misses        137       93      -44     

Impacted Files	Coverage Δ
src/linearmixedmodel.jl	98.41% <ø> (+3.02%)	⬆️
src/linalg.jl	98.03% <96.77%> (+35.21%)	⬆️
src/likelihoodratiotest.jl	98.52% <100.00%> (ø)
src/linalg/pivot.jl	100.00% <100.00%> (ø)

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[dmbates]

Thank you for doing this. Creating the pivoted_qr function was a good solution to avoid special-casing the code at every call to the decomposition.

I took the liberty of dropping the _zerocorr code while doing the code cleanup. You are welcome to change that if you don't think it belongs here.