Add deprecated 7->8 label to some settings

[parkertimmins]

Add the IndexSettingDeprecatedInV7AndRemovedInV8 label to the settings:

• index.soft_deletes.enabled
• index.translog.retention.size
• index.translog.retention.age

The latter two have a TODO: Remove this setting in 9.0. comment so this seems like the right thing to do. I am less confident about index.soft_deletes.enabled, but since soft deletes are always in enabled in v8 it seems we should remove the setting.

Some context:
While working on reindexing datastreams from v7 to v8, tried to copy what the upgrade assistant does to filter out old settings when its upgrading an index. These three settings are specifically filtered out when upgrading. index.translog.retention.size and index.translog.retention.age are only removed if index.soft_deletes.enabled is enabled. But since index.soft_deletes.enabled is always enabled in v8, all three should be removed.

[DaveCTurner]

The trouble is, these settings weren't deprecated in v7, we can't just pretend that they were. They'd need to be deprecated in v8 for removal in v9, but that Property doesn't exist yet.

[parkertimmins]

@DaveCTurner were they not deprecated in v7? There are some warnings on https://www.elastic.co/guide/en/elasticsearch/reference/7.17/index-modules-translog.html and https://www.elastic.co/guide/en/elasticsearch/reference/current/index-modules.html that made me think they were. Though I guess it needs to produce a deprecation warning and not just be in the docs?

[parkertimmins]

Follow-up questions: It still makes sense to manually filter them out of indices when upgrading a v7 index to v8 index, right? Like how is done here?

[DaveCTurner]

Yes they'd need to be deprecated in v7 in the sense of emitting a deprecation warning. But also maybe the more important fact is that they weren't removed in v8 (in the sense that you can set them on an index with a v8.x created version) and again we can't pretend that they were.