Newrelic `BillingCycle` property Enum is removed since `2024-03-01` version

### API Spec link

https://github.com/Azure/azure-rest-api-specs/blob/8ad58020cfafc5ace71d85b31272acf4d6b694d3/specification/newrelic/resource-manager/NewRelic.Observability/stable/2024-03-01/NewRelic.json

### API Spec version

2024-03-01

### Question/Query

Newrelic `BillingCycle` property is changed from Enum to string type since `2024-03-01` version, this might cause breaking changes for generated SDK .
2024-01-01 with enum
https://github.com/Azure/azure-rest-api-specs/blob/8ad58020cfafc5ace71d85b31272acf4d6b694d3/specification/newrelic/resource-manager/NewRelic.Observability/stable/2024-01-01/NewRelic.json#L2100-L2129

2024-03-01
https://github.com/Azure/azure-rest-api-specs/blob/8ad58020cfafc5ace71d85b31272acf4d6b694d3/specification/newrelic/resource-manager/NewRelic.Observability/stable/2024-03-01/NewRelic.json#L2755-L2758

### Environment

_No response_

Compare the BillingCycle definitions in the linked 2024-01-01 and 2024-03-01 versions of specification/newrelic/resource-manager/NewRelic.Observability/stable/NewRelic.json. First read the referenced lines and the surrounding model definitions; done means determining whether the type change is an unintended breaking change and documenting or correcting the specification accordingly.

Written by the indexing model from the issue text.