Set default SLM retention invocation time (#47604)

This adds a default for the `slm.retention_schedule` setting, setting it
to `0 30 1 * * ?` which is 1:30am every day.

Having retention unset meant that it would never be invoked and clean up
snapshots. We determined it would be better to have a default than never
to be run. When coming to a decision, we weighed the option of an
absolute time (such as 1:30am) versus a periodic invocation (like every
12 hours). In the end we decided on the absolute time because it has
better predictability and consistency than a periodic invocation, which
would rely on when the master node were elected or restarted.

Relates to #43663

server/src/main/java/org/elasticsearch/common/settings/Setting.java

@@ -1073,6 +1073,11 @@ public class Setting<T> implements ToXContentObject {
         return new Setting<>(new SimpleKey(key), null, s -> "", Function.identity(), validator, properties);
     }
 
+    public static Setting<String> simpleString(String key, String defaultValue, Validator<String> validator, Property... properties) {
+        validator.validate(defaultValue);
+        return new Setting<>(new SimpleKey(key), null, s -> defaultValue, Function.identity(), validator, properties);
+    }
+
     public static Setting<String> simpleString(String key, Setting<String> fallback, Property... properties) {
         return simpleString(key, fallback, Function.identity(), properties);
     }

x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/LifecycleSettings.java

@@ -38,7 +38,10 @@ public class LifecycleSettings {
 
     public static final Setting<Boolean> SLM_HISTORY_INDEX_ENABLED_SETTING = Setting.boolSetting(SLM_HISTORY_INDEX_ENABLED, true,
         Setting.Property.NodeScope);
-    public static final Setting<String> SLM_RETENTION_SCHEDULE_SETTING = Setting.simpleString(SLM_RETENTION_SCHEDULE, str -> {
+    public static final Setting<String> SLM_RETENTION_SCHEDULE_SETTING = Setting.simpleString(SLM_RETENTION_SCHEDULE,
+        // Default to 1:30am every day
+        "0 30 1 * * ?",
+        str -> {
         try {
             if (Strings.hasText(str)) {
                 // Test that the setting is a valid cron syntax