Drop support for unmaintained Elixir/Erlang versions

[unflxw]

Some Elixir and Erlang versions that we support are EOL. We should drop support for them at some point. Doing so would allow us to clean up lots of special casing for different versions, especially when declaring dependencies, using compile-time environment variables, or establishing SSL connections.

Dropping support should probably be done as a major version release, given that appsignal-elixir-plug and appsignal-elixir-phoenix declare a dependency on appsignal >= 2.0.x.

Here are draft PRs that do the cleanup mentioned above:

• Elixir: Remove unmaintained Elixir/Erlang versions #864
• Plug: Remove unmaintained Elixir/Erlang versions appsignal-elixir-plug#36
• Phoenix: Remove unmaintained Elixir/Erlang versions appsignal-elixir-phoenix#71

We need to decide when we want to do this. It seems that other libraries bumped the minimum version to Elixir 1.11 or Elixir 1.12 alongside with releasing support for Elixir 1.15 (which we already support) so we wouldn't be alone in this:

• Fix deprecation warnings for Elixir 1.15 (& require Elixir 1.12+) kbrw/sweet_xml#94
• Fix Elixir 1.15 warning, require Elixir 1.11+ & bump Ecto/Postgrex requirements pawurb/ecto_psql_extras#46
• Fix deprecation warning on Elixir 1.15, require Elixir 1.11, adapt CI open-api-spex/open_api_spex#550