Rename COPYING to LICENSE

[scpeters]

🦟 Bug fix

Fixes automatic detection of license type. Similar to gazebosim/gz-math#521.

Summary

The LICENSE file contained a copy of the stanze
used at the top of source code files, while the
actual license was in the COPYING file. So remove
the stanza and put the actual Apache 2.0 license text in LICENSE.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[codecov]

Codecov Report

Merging #1937 (ce40440) into ign-gazebo3 (940e5f3) will not change coverage.
The diff coverage is n/a.

❗ Current head ce40440 differs from pull request most recent head 7ab5764. Consider uploading reports for the commit 7ab5764 to get more accurate results

@@             Coverage Diff              @@
##           ign-gazebo3    #1937   +/-   ##
============================================
  Coverage        77.96%   77.96%           
============================================
  Files              255      255           
  Lines            15041    15041           
============================================
  Hits             11726    11726           
  Misses            3315     3315           

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