policy_id type in Get Snapshot Lifecycle API REST spec should be list

[russcam]

In line with other REST API specs that accept a comma-separated list of values, the slm.get_lifecycle REST API spec policy_id should be of type "list", and not "string":

elasticsearch/x-pack/plugin/src/test/resources/rest-api-spec/api/slm.get_lifecycle.json

Lines 14 to 19 in 603c3e6

	"parts":{
	"policy_id":{
	"type":"string",
	"description":"Comma-separated list of snapshot lifecycle policies to retrieve"
	}
	}

This affects REST specs back to v7.4.0.

The docs should also be updated to use policy_id instead of policy_ids, to match the REST API spec.

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/REST API)