Remove ubuntu1604 from presubmit.yml

Ubuntu 16.04 is end-of-life, we're going to remove it from Bazel CI.

If you like you can add testing on `ubuntu2004` platform which we also support.
diff --git a/.bazelci/presubmit.yml b/.bazelci/presubmit.yml
index 38ca793..a754f32 100644
--- a/.bazelci/presubmit.yml
+++ b/.bazelci/presubmit.yml
@@ -1,7 +1,7 @@
 ---
 tasks:
   ubuntu:
-    platform: ubuntu1604
+    platform: ubuntu1804
     test_targets:
     - //...
   macos: