tf-a-lts2.8-gerrit-tforg-l1/l2: Increase retention period

These are low-volume jobs, discard based on the number of builds rather
than dates.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I97d62fd9655adb149ae8460ea5d0adf3585cb0ba
diff --git a/tf-a-lts2.8-gerrit-tforg-l2.yaml b/tf-a-lts2.8-gerrit-tforg-l2.yaml
index cf4bc8a..4e2f1a2 100644
--- a/tf-a-lts2.8-gerrit-tforg-l2.yaml
+++ b/tf-a-lts2.8-gerrit-tforg-l2.yaml
@@ -13,7 +13,8 @@
         enabled: true
         max-total: 5
     - build-discarder:
-        days-to-keep: 14
+        days-to-keep: 90
+        num-to-keep: 15
     - authorization:
         !include: authorization.yaml.inc
     parameters: