Enable custom_webhook_template_serialization feature flag by default

What does this MR do?

Enables the custom_webhook_template_serialization feature flag by default.

Why?

The feature has been successfully rolled out to production (100% enabled since November 11, 2025 at 6:01:19 PM GMT+1) and has been running stably. This MR follows our standard feature flag lifecycle before the complete cleanup.

What changes?

• Sets default_enabled: true in the feature flag YAML
• Adds a changelog entry

What's next?

After running by default for 2-3 milestones, we'll proceed with the complete feature flag removal.

Related

• Introduced: !197992 (merged)
• Rollout issue: #560952 (closed)
• Related to #580455 (closed)