Set canonical_version in mike configuration

[jayqi]

Currently, any particular version of the docs has header metadata like

<link rel="canonical" href="https://cloudpathlib.drivendata.org/v0.16/authentication/">

that references itself.

This configuration change will cause it to instead set it to

<link rel="canonical" href="https://cloudpathlib.drivendata.org/stable/authentication/">

This will only change for future deploys going forward. I've tested this by manually doing a staged deploy for latest, and it seems to work, and I will manually edit existing deploys on the gh-pages branch. (See #399)

Closes #397

[github-actions]

🚀 Deployed on https://deploy-preview-398--gallant-agnesi-5f7bb3.netlify.app

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (991704a) 94.1% compared to head (498a952) 94.4%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #398     +/-   ##
========================================
+ Coverage    94.1%   94.4%   +0.3%     
========================================
  Files          22      22             
  Lines        1560    1560             
========================================
+ Hits         1468    1474      +6     
+ Misses         92      86      -6     

see 3 files with indirect coverage changes