Chore: Update License identifier in mix.exs

[nelsonic]

At present when we run mix hex.publish we see the following message:

The following licenses are not recognized by SPDX:
 * GNU GPL v2.0

Consider using licenses from https://spdx.org/licenses

Todo:

• [x] Update the license in mix.exs similar to: github.com/michalmuskala/jason/blob/master/mix.exs#L61

https://spdx.org/licenses/

[nelsonic]

PR: #189 assigned to @SimonLab ⏳

[nelsonic]

Thanks @SimonLab for quick review & merge. 🙌

No longer get the warning when attempting to publish:

 Version: 1.6.6
  Build tools: mix
  Description: Turnkey Auth Auth Application
  Licenses: GPL-2.0-or-later
  Links:
    GitHub: https://github.com/dwyl/auth
  Elixir: ~> 1.12
Before publishing, please read the Code of Conduct: https://hex.pm/policies/codeofconduct

Publishing package to public repository hexpm.

Proceed? [Yn] y

[SimonLab]

Thanks for the update. I wasn't sure what spdx means: https://en.wikipedia.org/wiki/Software_Package_Data_Exchange:

[nelsonic]

Yeah, I didn't know either. Googled it. Forgot to share the link. Thanks @SimonLab 🙌