[elixir] ci: bump OTP to 28.5.0.2

[nicolazar]

Purpose

This PR fixes the CI for Elixir bindings.

Brief change log

The CI fails with:

Could not start Hex. Try fetching a new version with "mix local.hex" or uninstalling it with "mix archive.uninstall hex.ez"
** (MatchError) no match of right hand side value
{:failed_to_start_child, Hex.State,
         {:undef,
          [
            {:re, :import,
             [
               {:re_exported_pattern,
               ...

The problem here is essentially a toolchain version mismatch: the latest version of hex - 2.5.0 (picked by the CI action) had added a new feature that uses a regex for a duration parser, but compiled under Elixir 1.19 on OTP 28, that regex is serialized in OTP 28's new export form and reconstructed at load time via re:import/1. However, this function only since OTP 28.1 (hence the undef).

So bumping the OTP version (latest 28.x) should fix this issue.

[fresh-borzoni]

@nicolazar Thank you for the PR, makes sense 👍