docs: rename LICENSE.txt to LICENSE

[tas50]

The README links [LICENSE](LICENSE), but the file is LICENSE.txt — so that link 404s on GitHub today. Renaming the file fixes it, and matches the rest of the test-kitchen plugins: 18 of 21 already use LICENSE, and kitchen-openstack was moved over in #258.

.yardopts listed the old path as an extra file, so it is updated to match.

The gemspec builds spec.files from git ls-files, so it needs no change and the built gem still ships the license — verified below.

Verification

$ bundle exec cookstyle --chefstyle
8 files inspected, no offenses detected

$ bundle exec rake test
85 examples, 0 failures

$ gem build *.gemspec && tar -xOf *.gem data.tar.gz | tar -tz | grep '^LICENSE$'
LICENSE

Not fixed here

yard emits two warnings against the license file:

[warn]: Cannot resolve link to yyyy from text:
[warn]: Cannot resolve link to name from text:

Those come from the Apache boilerplate's [yyyy] and [name of copyright owner] placeholders being parsed as markdown links. They are unaffected by the rename — I confirmed they persist with the new filename — and fixing them is a separate decision about whether the license belongs in the YARD extra-files list at all.