[MEMO for ME] restore threshold setting for mix test coverage

[takasehideki]

ec176d6 decreased the threshold setting for mix test --cover to 80% (default is 90%).

zenohex/mix.exs

Lines 97 to 98 in ec176d6

	# WHY: see https://github.com/biyooon-ex/zenohex/issues/77
	summary: [threshold: 80]

The cause may be the issue in Elixir v1.17. As a result, our CI could not pass because of the coverage.
Since this has already been fixed but has not yet been taken in the public release, we will restore this setting (remove lines) after Elixir v1.17.4 is released.