Summary:
Turning on vector index background compactions on by default since these changes are ready:
D47937  `[#29151] docdb: Implement PriorityThreadPoolToken to control parallel execution`
D47941  `[#27601] docdb: Limit number of vector index compactions per tserver`
Jira: DB-18979

Test Plan: Jenkins

Reviewers: rthallam

Reviewed By: rthallam

Subscribers: yql, ybase

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D47963