fix lowerbd(::MixedModel) to work as singularity reference check

[palday]

• add entry in NEWS.md
• after opening this PR, add a reference and run docs/NEWS-update.jl to update the cross-references.
• I've bumped the version appropriately

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.60%. Comparing base (ec7b986) to head (b323260).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #864      +/-   ##
==========================================
+ Coverage   95.46%   95.60%   +0.13%     
==========================================
  Files          38       38              
  Lines        3682     3682              
==========================================
+ Hits         3515     3520       +5     
+ Misses        167      162       -5     

Flag	Coverage Δ
current	95.29% <100.00%> (+0.13%)	⬆️
minimum	95.54% <100.00%> (+0.08%)	⬆️
nightly	95.26% <100.00%> (+0.13%)	⬆️

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[dmbates]

Looks good. Thanks for doing this.