Move Terraform state versioning default to database

What does this MR do?

Changes the default of terraform_states.versioning_enabled from false -> true. This was previously defined in application code (while we migrated to supporting versioning), so doesn't cause a change in behaviour.

Migration output

== 20201119213406 ChangeTerraformVersioningEnabledDefault: reverting ==========
-- change_column_default(:terraform_states, :versioning_enabled, {:from=>true, :to=>false})
   -> 0.0045s
== 20201119213406 ChangeTerraformVersioningEnabledDefault: reverted (0.0080s) =

== 20201119213406 ChangeTerraformVersioningEnabledDefault: migrating ==========
-- change_column_default(:terraform_states, :versioning_enabled, {:from=>false, :to=>true})
   -> 0.0038s
== 20201119213406 ChangeTerraformVersioningEnabledDefault: migrated (0.0039s) =

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content