Remove support for array of strings when defining services for Docker Executor in favor of array of tables

We currently implement UnmarshalTOML for DockerService to allow the now legacy array of strings and the new array of tables syntaxes when declaring services. In %1.30 we should fallback to the default toml parsing by removing this function.

Before:

services = ["mysql:latest", "postgres:latest"]

After:

[[runners.docker.services]]
  name = "mysql:latest"
  alias = "db"

[[runners.docker.services]]
  name = "redis:latest"
  alias = "cache"

Related to !1673 (merged)