Point introduced_by_url to correct MR for `pi_ci_builds`

What does this MR do and why?

!120873 (merged) did not set introduced_by_url on db/docs/p_ci_builds.yml. As a consequence the following diff is now generated when running migrations - note the trailing whitespace:

diff --git a/db/docs/p_ci_builds.yml b/db/docs/p_ci_builds.yml
index 2ad98d934858..d619b74e71a3 100644
--- a/db/docs/p_ci_builds.yml
+++ b/db/docs/p_ci_builds.yml
@@ -9,6 +9,6 @@ classes:
 feature_categories:
 - continuous_integration
 description: Routing table for ci_builds
-introduced_by_url:
+introduced_by_url: 
 milestone: '16.1'
 gitlab_schema: gitlab_ci

This MR sets the introduced_by_url to the correct MR.

Related to #382033 (closed)