[Search Pipelines] Remove Experimental Flag

[macohen]

I don't think this is the typical process, but I'd like to suggest that in the 2.9 release we update https://github.com/opensearch-project/OpenSearch/blob/main/server/src/main/java/org/opensearch/common/util/FeatureFlags.java instead of just removing the feature flag itself. So, we would update:

public static final Setting<Boolean> SEARCH_PIPELINE_SETTING = Setting.boolSetting(SEARCH_PIPELINE, false, Property.NodeScope);

to

public static final Setting<Boolean> SEARCH_PIPELINE_SETTING = Setting.boolSetting(SEARCH_PIPELINE, true, Property.NodeScope);

The advantage is that we minimize change in the first release and then remove the flag in 2.10. Of course, I may not understand how the feature flags work in the first place and we can only remove the flag instead of setting it to true first.

[macohen]

cc: @mingshl, @noCharger, @sejli

[noCharger]

The AI to fully remove the feature flag:

• Keep the setting, but make it true by default. (Done v2.9.0)
• Backport that to 2.x (Done v2.9.0)
• Stop enabling the setting for mixed cluster tests (because it is enabled on main and 2.x). (Target next release)
• Remove the setting.(Target next release)