ELSER model should be enabled/true by default for Platinum edition

[ajainghub]

When selecting and deploying Elasticsearch Platinum edition (e.g. in the RAG DA Stack), make ELSER model enabled/true by default.

Also please confirm the Elasticsearch DA deploys this elser model: .elser_model_2_linux-x86_64

[akocbek]

added in terraform-ibm-modules/dev-rag#38