Move piyush-garg to alumni

piyush-garg has had near-zero CLI activity in the last 12 months
(0 PRs, only 10 comments). DevStats shows 17 CLI contributions
(mostly old commits) vs 976 contributions org-wide in the last
2 years — activity dropped off completely.

See the full triage report:
https://gist.github.com/vdemeester/c7ce8d4fea6f7a2f9b3f685558ce8ded

/kind cleanup

Author: vdemeester

OWNERS

@@ -4,7 +4,6 @@ approvers:
 - divyansh42
 - vdemeester
 - chmouel
-- piyush-garg
 - vinamra28
 - pratap0007
 
@@ -17,3 +16,4 @@ reviewers:
 # danielhelfand
 # hrishin
 # sthaha
+# piyush-garg