Add an SPDX-License-Identifier to all source files and update copyright year.

[JSorngard]

No description provided.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.32%. Comparing base (406377b) to head (341183a).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #148      +/-   ##
==========================================
- Coverage   89.37%   88.32%   -1.05%     
==========================================
  Files          11       11              
  Lines        1007      831     -176     
==========================================
- Hits          900      734     -166     
+ Misses        107       97      -10     

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[JSorngard]

The reduction in code coverage seems to be an artifact, given that this PR does not touch any source code at all and only modifies the license files or adds comments.