Fix deprecation warnings based on 1.14-dev.

[crova]

Greetings, how is it going?

As mentioned on this issue there are a few warnings when compiling while using elixir 1.14.

Besides those that were mentioned, when useing Guardian on a module, the following warn would raise:

warning: Application.get_env/3 is discouraged in the module body, use Application.compile_env/3 instead

This PR address both issues.

Please let me know if I should improve this PR somehow.

Cheers.

[gungorkocak]

I'm also trying to upgrade Elixir v1.14.0-rc.1 and facing the same deprecation warning with Application.get_env/3.

Also mix compile --force --warnings-as-errors is a requirement for me(i.e. CI pipeline).
That ties upgrading Elixir to this PR (or a better solution).

[kevinkirkup]

Related to #704

[yordis]

💜