Deprecate the xpack.task_manager.claim_strategy setting

[mikecote]

The xpack.task_manager.claim_strategy setting was introduced to feature flag the new mget claim strategy and eventually turn into a fallback mechanism if ever a cluster was experiencing issues with mget. Now that we're rolling out mget as the default to everyone in 8.17, we should deprecate in 9.0 the setting to discourage its usage, given we'll be fixing issues, if any, as they arise.

In a future 9.x version, we plan to make this setting a no-op and remove the update_by_query task claimer.

Definition of Done

• Change applied to 9.0+ only
• Configuration setting deprecated using the deprecations object
• Message to ask users to remove this setting as a fix
• Tests

[elasticmachine]

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