Remove ubuntu1604 from presubmit.yml (#6715)

* 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.

* Add ubuntu2004 to presubmit.yml

.bazelci/presubmit.yml

@@ -1,18 +1,18 @@
 ---
 buildifier: latest
 platforms:
-  ubuntu1604:
-    build_targets:
-    - "..."
-    test_targets:
-    - "..."
   ubuntu1804:
     build_targets:
-    - "..."
+    - "//..."
     test_targets:
-    - "..."
+    - "//..."
+  ubuntu2004:
+    build_targets:
+    - "//..."
+    test_targets:
+    - "//..."
   macos:
     build_targets:
-    - "..."
+    - "//..."
     test_targets:
-    - "..."
+    - "//..."