[Fleet] Use same action_id for all batches in an upgrade

[nchaulet]

Summary

Resolve #135801

The .fleet-actions index was build to allow the same action_id for different document, it's already used by osquery. We can use the same action_id for all the batches of an upgrade.

How to test?

1. create a bunch of upgradeable agents
2. Trigger an upgrade with a small batchSize

POST kbn:/api/fleet/agents/bulk_upgrade
{
  "agents": "*",
  "version": "8.4.0-SNAPSHOT",
  "batchSize": 1
}

3. You can see there is multiple fleet actions (one per agent)
4. but only one upgrade toast

@juliaElastic if it make sense to you I can add some automated test for that.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[kibana-ci]

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Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @nchaulet

[kpollich]

Makes sense to me.

[juliaElastic]

@nchaulet thanks for fixing, I didn't know it is already supported

[juliaElastic]

🚀