refactor(observability): increase the character limit for alertgroup's rule.expression (#1292)

Increase the limit to match the updated limit on the api

https://docs.api.stackit.cloud/documentation/argus/version/v1#tag/alert-groups/operation/v1_projects_instances_alertgroups_create (v1.1.1)

Co-authored-by: Alexander Dahmen <alexander.dahmen@inovex.de>

Author: umarhussain15

stackit/internal/services/observability/alertgroup/resource.go

@@ -227,7 +227,7 @@ func (a *alertGroupResource) Schema(_ context.Context, _ resource.SchemaRequest,
 							Description: descriptions["expression"],
 							Required:    true,
 							Validators: []validator.String{
-								stringvalidator.LengthBetween(1, 600),
+								stringvalidator.LengthBetween(1, 2000),
 								// The API currently accepts expressions with trailing newlines but does not return them,
 								// leading to inconsistent Terraform results. This issue has been reported to the Obs team.
 								// Until it is resolved, we proactively notify users if their input contains a trailing newline.