Bump Hex Elixir to 1.15

[TylerWitt]

What are you trying to accomplish?

Bumps Elixir to 1.15.8 within Hex.

There's a few notable changes due to the upgrade.

1. When Mix tries to process remote_convergers (Hex.RemoteConverger), Hex.Mix and other nested hex modules weren't loaded.
2. The term -> binary -> term message passing from run.exs to the other scripts now has Base.encode64/decode64 calls in order to make sure the binaries can be processed by IO.write
3. :hex and :ssh are added as an extra_applications
4. The deps_warning mixfile needed a new kind of warning for the associated test to still pass. The old mixfile raises an error due to the missing parens on deps now.

Checklist

• I have run the complete test suite to ensure all tests and linters pass.
• I have thoroughly tested my code changes to ensure they work as expected, including adding additional tests for new functionality.
• I have written clear and descriptive commit messages.
• I have provided a detailed description of the changes in the pull request, including the problem it addresses, how it fixes the problem, and any relevant details about the implementation.
• I have ensured that the code is well-documented and easy to understand.

[TylerWitt]

This closes #8622

[TylerWitt]

I also have PRs on my fork for 1.16, 1.17, and 1.18 (separately).

Is that the preference? Or is it better to just jump to 1.18?

I figure it would be good to keep the blames correct and the separation helps in case there is any reversions needed.

[abdulapopoola]

Thanks @TylerWitt , we're fine doing a sequential rollout for clarity.

[grzuy]

@TylerWitt Thanks for all your work on updating Elixir/OTP for dependabot 🙏

Reporting back on an issue having after these changes...

Apparently getting "\n" out of the encoded output of parse_deps.exs and raising here when trying to decode back?

Thanks!

[TylerWitt]

Can you raise an issue and post the full stacktrace? Thanks!

It's probably that errors need to also be encoded now.