Update for 2.26 compatibility

[andrjohns]

This PR updates your package for compatibility with the next version of rstan

[codecov-commenter]

Codecov Report

Base: 77.39% // Head: 77.44% // Increases project coverage by +0.04% 🎉

Coverage data is based on head (02d82ab) compared to base (322e628).
Patch has no changes to coverable lines.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main       #1      +/-   ##
==========================================
+ Coverage   77.39%   77.44%   +0.04%     
==========================================
  Files          22       22              
  Lines        2442     2447       +5     
==========================================
+ Hits         1890     1895       +5     
  Misses        552      552              

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[andrjohns]

Would you be able to merge this PR and release to CRAN? We're aiming to submit a new release of rstan soon

[dcgerard]

Sorry. I'm on parental leave. I'll try to do that today.

Thanks for the PR.

[dcgerard]

Update submitted to CRAN. Thanks again.