…lastic#149504)

## Summary

Closes elastic#147605

### Description
Clusters that exist since pre-8.0 might have data streams that have
`managed_by: ingest_manager` in their` _meta` properties, instead of the
current value `managed_by: fleet`.
However, with the merge of
elastic#143300, the data streams view
filters out any data streams that don't have `managed_by: fleet`
metadata, so when updating from 7.x to 8.6.x no data streams are
returned.

### Solution
It was decided to simply add an additional check in the data stream
handler to allow for "legacy" metadata, and to avoid doing migrations
that can be dangerous for the users data.

### Testing
I'm looking for a way to reliably test this locally - I've only managed
to reproduce it on cloud

(cherry picked from commit 49ff27e)

# Conflicts:
#	x-pack/plugins/fleet/server/routes/data_streams/handlers.ts