Update mongo

Changes:

- https://github.com/docker-library/mongo/commit/3f9f4cd: Update to 4.0.21
- https://github.com/docker-library/mongo/commit/847dd2d: Doh.

library/mongo

@@ -25,24 +25,24 @@ GitCommit: d7b81e278d4b1f5e8c3011e397d4f918070aea3a
 Directory: 3.6/windows/windowsservercore-1809
 Constraints: windowsservercore-1809
 
-Tags: 4.0.20-xenial, 4.0-xenial
-SharedTags: 4.0.20, 4.0
+Tags: 4.0.21-xenial, 4.0-xenial
+SharedTags: 4.0.21, 4.0
 # see http://repo.mongodb.org/apt/ubuntu/dists/xenial/mongodb-org/4.0/multiverse/
 Architectures: amd64, arm64v8
-GitCommit: f79f35ce16c972632c38f74d1e70b4cdb419f2ed
+GitCommit: 3f9f4cdaf6c6a64bf0ad32c7d10aa2029873f585
 Directory: 4.0
 
-Tags: 4.0.20-windowsservercore-ltsc2016, 4.0-windowsservercore-ltsc2016
-SharedTags: 4.0.20-windowsservercore, 4.0-windowsservercore, 4.0.20, 4.0
+Tags: 4.0.21-windowsservercore-ltsc2016, 4.0-windowsservercore-ltsc2016
+SharedTags: 4.0.21-windowsservercore, 4.0-windowsservercore, 4.0.21, 4.0
 Architectures: windows-amd64
-GitCommit: 48df1464cd062f431183c0447d08c720d1947ae6
+GitCommit: 3f9f4cdaf6c6a64bf0ad32c7d10aa2029873f585
 Directory: 4.0/windows/windowsservercore-ltsc2016
 Constraints: windowsservercore-ltsc2016
 
-Tags: 4.0.20-windowsservercore-1809, 4.0-windowsservercore-1809
-SharedTags: 4.0.20-windowsservercore, 4.0-windowsservercore, 4.0.20, 4.0
+Tags: 4.0.21-windowsservercore-1809, 4.0-windowsservercore-1809
+SharedTags: 4.0.21-windowsservercore, 4.0-windowsservercore, 4.0.21, 4.0
 Architectures: windows-amd64
-GitCommit: 48df1464cd062f431183c0447d08c720d1947ae6
+GitCommit: 3f9f4cdaf6c6a64bf0ad32c7d10aa2029873f585
 Directory: 4.0/windows/windowsservercore-1809
 Constraints: windowsservercore-1809