Update Elixir and dependencies

[Annopaolo]

• Update Elixir to 1.15 (both in app and GH workflows)
• Contextually, update ssl_verify_fun dependency to 1.17 to make it work again on Elixir 1.15
• Write charlists as ~c"charlist" instead of 'charlist' to pacify mix.format on Elixir 1.15
• Update astarte_rpc and astarte_core dependencies to latest master version

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (e5f4eb1) 76.66% compared to head (6a800a8) 76.66%.

Files	Patch %	Lines
lib/astarte_vmq_plugin.ex	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #82 +/- ## ======================================= Coverage 76.66% 76.66% ======================================= Files 13 13 Lines 330 330 ======================================= Hits 253 253 Misses 77 77 ```

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