Alerting: Enable group-level rule evaluation jittering by default, remove feature toggle

[alexweav]

What is this feature?

This PR removes the jitterAlertRules feature toggle. The new default behavior is as if it's enabled.
The jitter behavior was introduced in #80766

The effect is that the evaluation time of rule groups are distributed over their group interval. Rules within a group will still evaluate together on all replicas, but this time is smoothed out over the rule's evaluation interval rather than synchronized with all other rules on the entire system.

This feature has been globally active in Grafana Cloud for a while now, without issue. It heavily reduces load on datasources by decreasing query spikes. The lack of spiky evaluations improves alert evaluation time and notification p99 time by roughly 10% across the board.

The jitter is now on by default, but can still be disabled with disable_jitter under the [unified_alerting] config section.

Which issue(s) does this PR fix?:

Fixes #53744

Special notes for your reviewer:

Please check that:

• It works as expected from a user's perspective.
• If this is a pre-GA feature, it is behind a feature toggle.
• The docs are updated, and if this is a notable improvement, it's added to our What's New doc.