Add @Yongxuanzhang as a pipelines maintainer

@Yongxuanzhang has been a consistent contributor to pipelines for a long time now and has worked on a number of big features such as trusted resources and array parameters. He has also reviewed 130+ PRs and meets all the other criteria for being a maintainer.

Reviews: https://github.com/tektoncd/pipeline/pulls?q=is%3Apr+yongxuanzhang+is%3Aclosed

OWNERS_ALIASES

@@ -10,6 +10,7 @@ aliases:
   - lbernick
   - abayer
   - wlynch
+  - yongxuanzhang
 
   apis-approvers:
   - afrittoli