Rename COPYING to LICENSE (#1252)

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.

Similar to gazebosim/gz-math#521.

Signed-off-by: Steve Peters <[email protected]>

Author: scpeters