Fix Apache license version (backport to citadel)

[scpeters]

Backport #325 to citadel. Use rebase-and-merge.

[codecov]

Codecov Report

Merging #326 (a6048cf) into ign-physics2 (7a1c20d) will not change coverage.
The diff coverage is n/a.

❗ Current head a6048cf differs from pull request most recent head dcd0845. Consider uploading reports for the commit dcd0845 to get more accurate results

@@              Coverage Diff              @@
##           ign-physics2     #326   +/-   ##
=============================================
  Coverage         82.84%   82.84%           
=============================================
  Files               108      108           
  Lines              4243     4243           
=============================================
  Hits               3515     3515           
  Misses              728      728           

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