downsize the build config max number

Signed-off-by: Karl Zhang <karl.zhang@arm.com>
Change-Id: I1a2486a04438604a5263505b158fe2c8861356c8
diff --git a/tf-m-build-config.yaml b/tf-m-build-config.yaml
index abce7ff..4581ae2 100644
--- a/tf-m-build-config.yaml
+++ b/tf-m-build-config.yaml
@@ -14,7 +14,7 @@
     properties:
       - build-discarder:
           days-to-keep: 15
-          num-to-keep: 6000
+          num-to-keep: 3000
       - authorization:
           anonymous:
             - job-read