Rollout client-side updates w/ mget as the default task claimer

[mikecote]

Part of #155770

Blocked on https://github.com/elastic/response-ops-team/issues/137

Feature Description

Change the task manager task claiming algorithm to use a _search to retrieve candidate tasks, a _mget to prune the docs whose version number doesn't match, and then a _bulk to claim the tasks. This will increase the background task capacity in Serverless.

Business Value

Increased background task capacity, reducing the COGS for running alerting rules and actions, and providing a lower MTTD/MTTR.

Rollout strategy (with bake periods in between)

1. Select internal serverless projects have the new task claiming logic enabled
2. Turn on for all canary serverless projects
3. Turn on for select non canary serverless projects
4. Turn on for all serverless projects
5. Turn on for ESS and on-prem

Definition of Done

• Rollout is done in a gradual manner
• Client-side updates w/ mget to prune stale docs is the default way tasks are claimed

[elasticmachine]

Pinging @elastic/response-ops (Team:ResponseOps)

[mikecote]

Rollout to serverless is complete, we are onboarding 12.5% of ECH clusters in 8.16 + internal clusters and by 8.17 we'll have this enabled by default via #194625. Closing..