Rename Erlang Ecosystem to Hex

[maennchen]

To publish advisories with packages regarding hex.pm, currently the ecosystem has to be set to "Erlang". Since hex.pm is for Erlang, Elixir, Gleam and more, this is confusing.

Additionally, the resulting purl will be of type hex and the OSV package ecosystem is also set to "Hex".

It therefore would make sense, to rename "Erlang" to "Hex" everywhere in the GitHub Advisory pages / forms.

~~Alternatively, something like "BEAM" would also work, since it describes the VM all those languages share.~~ - Let's not do that since we plan to have another purl type for OTP applications that are preinstalled: https://security.erlef.org/specs/otp_purl_type