Bump the min supported bazel version (#1414)

To use config.string with allow_multiple
diff --git a/.bazelci/presubmit.yml b/.bazelci/presubmit.yml
index c441c65..0e45f50 100644
--- a/.bazelci/presubmit.yml
+++ b/.bazelci/presubmit.yml
@@ -149,13 +149,13 @@
     bazel: "rolling"
   ubuntu1804:
     name: "Min Bazel Version"
-    bazel: "4.0.0"
+    bazel: "5.0.0"
     platform: ubuntu1804
     build_targets: *default_linux_targets
     test_targets: *default_linux_targets
   ubuntu1804_with_aspects:
     name: "Min Bazel Version With Aspects"
-    bazel: "4.0.0"
+    bazel: "5.0.0"
     platform: ubuntu1804
     build_targets: *default_linux_targets
     test_targets: *default_linux_targets