[Bug] elasticstack_kibana_alerting_rule interval doesn't accept days

[reakaleek]

Describe the bug
Using days in the elasticstack_kibana_alerting_rule interval option leads to an error.

To Reproduce

I imported an existing Kibana alert rule where the check interval is 7 days using terraform import

When run terraform plan

I get an expected diff with:

...
# elasticstack_kibana_alerting_rule.test-reporter will be updated in-place
  ~ resource "elasticstack_kibana_alerting_rule" "test-reporter" {
        id                    = "default/b6679cfe-e09a-4114-82ba-f1684563ee4a"
      ~ interval              = "7d" -> "1h"
...

But when I change the interval to "7d" in the tf file, I get the following error.

╷
│ Error: "interval" contains an invalid duration: time: unknown unit "d" in duration "7d"
│ 
│   with elasticstack_kibana_alerting_rule.test-reporter,
│   on 02-kibana-alert-rules.tf line 55, in resource "elasticstack_kibana_alerting_rule" "test-reporter":
│   55:   interval     = "7d"
│ 
╵

Expected behavior
I should be able to plan or apply with the interval "7d"

Debug output
Run terraform command with TF_LOG=trace and provide extended information on TF operations. Please ensure you redact any base64 encoded credentials from your output.
eg

[DEBUG] provider.terraform-provider-elasticstack_v0.11.0: Authorization: Basic xxxxx==

Screenshots
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Versions (please complete the following information):

• OS: macos 14.6.1
• Terraform Version: v1.9.3
• Provider version 0.11.6
• Elasticsearch Version 8.15.1

Additional context
Add any other context about the problem here.

[tobio]

Related to #829