Update SLM Retention serialization versions

[dakrone]

With #46506 backporting SLM retention, this updates master's version
guards for SLM retention.

Relates to #43663

[elasticmachine]

Pinging @elastic/es-core-features

[gwbrown]

@dakrone just noticed in going to verify that this hit all the right places that we still need version guards in SnapshotLifecyclePolicy's writeTo:

elasticsearch/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/slm/SnapshotLifecyclePolicy.java

Lines 267 to 274 in 56aabcd

	public void writeTo(StreamOutput out) throws IOException {
	out.writeString(this.id);
	out.writeString(this.name);
	out.writeString(this.schedule);
	out.writeString(this.repository);
	out.writeMap(this.configuration);
	out.writeOptionalWriteable(this.retentionPolicy);
	}

And StreamInput constructor:

elasticsearch/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/slm/SnapshotLifecyclePolicy.java

Lines 99 to 106 in 56aabcd

	public SnapshotLifecyclePolicy(StreamInput in) throws IOException {
	this.id = in.readString();
	this.name = in.readString();
	this.schedule = in.readString();
	this.repository = in.readString();
	this.configuration = in.readMap();
	this.retentionPolicy = in.readOptionalWriteable(SnapshotRetentionConfiguration::new);
	}

[gwbrown]

LGTM now