tf-m-release: Set reasonable num-to-keep value

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: Idfe9b9c5510d5f08408a12b3b37295331f36fce3
diff --git a/tf-m-release.yaml b/tf-m-release.yaml
index ac6f92e..79c2299 100644
--- a/tf-m-release.yaml
+++ b/tf-m-release.yaml
@@ -17,7 +17,7 @@
     properties:
       - build-discarder:
           days-to-keep: 30
-          num-to-keep: 3
+          num-to-keep: 20
     parameters:
       - string:
           name: 'FILTER_GROUP'