[Fleet] Deprecate expiration option in POST /enrollment_api_keys

[jillguyonnet]

Update: I realised that the Security API does allow setting the key expiration on creation (no expiry by default). If the intent is to have the Fleet API reflect that, then we should keep this option and pass it to the Security API instead. I didn't find another issue that mentions this.

The request schema for the POST /enrollment_api_keys endpoint allows an optional expiration option:

kibana/x-pack/plugins/fleet/server/types/rest_spec/enrollment_api_key.ts

Lines 50 to 55 in 7f22ca5

	export const PostEnrollmentAPIKeyRequestSchema = {
	body: schema.object({
	name: schema.maybe(schema.string()),
	policy_id: schema.string(),
	expiration: schema.maybe(schema.string()),
	}),

However, this option is never used in the backend:

kibana/x-pack/plugins/fleet/server/services/api_keys/enrollment_api_key.ts

Lines 208 to 217 in 7f22ca5

	export async function generateEnrollmentAPIKey(
	soClient: SavedObjectsClientContract,
	esClient: ElasticsearchClient,
	data: {
	name?: string;
	expiration?: string;
	agentPolicyId: string;
	forceRecreate?: boolean;
	}
	): Promise<EnrollmentAPIKey> {

It is also not possible to specify an expiration when creating an enrollment token in the UI:

If this option is deprecated, perhaps it could be removed altogether as part of the upcoming API breaking changes: #189746

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[jillguyonnet]

A detail that could be addressed as part of this: the log for enrollment token creation currently reads:

Creating enrollment API key [object Object]

Edit: fixed in #191807