Fix compilation on Elixir 1.19

[axelson]

Fixes this warning:

warning: setting :preferred_cli_env in your mix.exs "def project" is deprecated, set it inside "def cli" instead:

def cli do
  [preferred_envs: [coveralls: :test, "coveralls.html": :test, "coveralls.json": :test]]
end

(mix 1.19.1) lib/mix/cli.ex:189: Mix.CLI.preferred_cli_env/3
(mix 1.19.1) lib/mix/cli.ex:172: Mix.CLI.maybe_change_env_and_target/2
(mix 1.19.1) lib/mix/cli.ex:59: Mix.CLI.proceed/1
/Users/jason/.asdf/installs/elixir/1.19.1-otp-28/bin/mix:7: (file)
(elixir 1.19.1) src/elixir_compiler.erl:81: :elixir_compiler.dispatch/4
(elixir 1.19.1) src/elixir_compiler.erl:56: :elixir_compiler.compile/4

This works with at least Elixir 1.16.0-otp-26 based on CI 👍
After this is merged we should make the Elixir 1.19 check required for PRs