Update Backdrop to 1.21.4

This is the update to the latest release of Backdrop, 1.21.4. 

The full release can be found here:
https://github.com/backdrop/backdrop/releases/tag/1.21.4
Here is the commit log of the release
https://github.com/backdrop-ops/backdrop-docker/commits/master

library/backdrop

@@ -4,12 +4,12 @@ Maintainers: Mike Pirog <[email protected]> (@pirog),
              Greg Netsas <[email protected]> (@klonos)
 GitRepo: https://github.com/backdrop-ops/backdrop-docker.git
 
-Tags: 1.21.3, 1.21, 1, 1.21.3-apache, 1.21-apache, 1-apache, apache, latest
+Tags: 1.21.4, 1.21, 1, 1.21.4-apache, 1.21-apache, 1-apache, apache, latest
 Architectures: amd64, arm64v8
-GitCommit: cb71efedce22392184d6d10aa67109e8f7e86725
+GitCommit: 91ac0ee1e56bbff6d2fb49218b38a9cbf9d1a356
 Directory: 1/apache
 
-Tags: 1.21.3-fpm, 1.21-fpm, 1-fpm, fpm
+Tags: 1.21.4-fpm, 1.21-fpm, 1-fpm, fpm
 Architectures: amd64, arm64v8
-GitCommit: cb71efedce22392184d6d10aa67109e8f7e86725
+GitCommit: 91ac0ee1e56bbff6d2fb49218b38a9cbf9d1a356
 Directory: 1/fpm