Update keyspace to not be required for db creation

[sriramg96]

Recently we merged some changes to Astra backend which now allow the user to not require to pass a keyspace upon an astra db creation. Instead, if a keyspace is not passed then a keyspace named default_keyspace is created for the db instead. This applies for serverless, classic and db_type vector too for now

See https://github.com/riptano/astra-docs/pull/715/files for the documentation update to see how the dev-ops-api has been updated.

We should update the astra terraform provider documentation similarly, so that keyspace is now optional for create operations. https://registry.terraform.io/providers/datastax/astra/latest/docs/resources/database