fix: elastic indices datasource boolean parsing

[jdesnoes]

Hello,

A little fix on the elastic indices datasource. When parsing boolean settings the following error could occur:

Planning failed. Terraform encountered an error while generating this plan.

╷
│ Error: failed to convert setting to bool
│ 
│   with data.elasticstack_elasticsearch_indices.existing_indices["my-indice"],
│   on main.tf line 56, in data "elasticstack_elasticsearch_indices" "existing_indices":
│   56: data "elasticstack_elasticsearch_indices" "existing_indices" {
│ 
│ expected setting to be a bool but got %!t(string=false)

@tobio I think a fast review could let it be included in the 0.11.9 version.

Thanks.

[tobio]

Thanks @jdesnoes, are you able to add a changelog entry here?

[jdesnoes]

@tobio it looks there is failing test step on 8.8.2 but I don't see anything related to the changes I did with this PR.
I guess everything should be OK if we try to run it again?

[tobio]

Thanks for coming back to fix this up!