Summary:
Change the default for yb_enable_parallel_scan_colocated GUC variable to
false.

Test Plan:
Jenkins: rebase: 2026.1.0
Jenkins: urgent
Jenkins: test regex: TestPgRegressParallel

Reviewers: patnaik.balivada, smishra

Reviewed By: patnaik.balivada, smishra

Subscribers: yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D54886