DOC: only allow one canonical_link attribute

[FirefoxMetzger]

Closes: #703

Changes required="*" to requires="0" for //model/@canonical_link.

[codecov-commenter]

Codecov Report

Merging #716 (064acb6) into sdf11 (8b12338) will not change coverage.
The diff coverage is n/a.

❗ Current head 064acb6 differs from pull request most recent head 9debfb6. Consider uploading reports for the commit 9debfb6 to get more accurate results

@@           Coverage Diff           @@
##            sdf11     #716   +/-   ##
=======================================
  Coverage   88.24%   88.24%           
=======================================
  Files          73       73           
  Lines       11014    11014           
=======================================
  Hits         9719     9719           
  Misses       1295     1295           

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[azeey]

I think this can actually go into sdf9. I'll retarget it.

[azeey]

Actually, since this also changes sdf/1.8/model.sdf, we can just backport this to sdf9 once it's merged.

[osrf-triage]

This pull request has been mentioned on Gazebo Community. There might be relevant details there:

https://community.gazebosim.org/t/new-ignition-releases-2022-03-25-fortress-edifice-citadel/1343/1