Update mix.exs

Deps weren't getting updates even though a later release was available. Some of them were giving this warning:
Warning: call to crypto:rand_bytes/1 will fail, since it was removed in 20.0; use crypto:strong_rand_bytes/1

Now only cowlib will give that error. I tried updating cowboy, but then these errors show up in the logs and the instance can't be reached:
[error] Ranch protocol :error of listener Pleroma.Web.Endpoint.HTTP (cowboy_protocol) terminated
** (exit) :undef