Update version information to v26.3.1

[github-actions]

Resolves #881.

[haeter525]

LGTM.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.56%. Comparing base (7bf6773) to head (6346b25).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #882   +/-   ##
=======================================
  Coverage   81.56%   81.56%           
=======================================
  Files          80       80           
  Lines        6920     6920           
=======================================
  Hits         5644     5644           
  Misses       1276     1276           

Flag	Coverage Δ
unittests	81.56% <100.00%> (ø)

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