Merge pull request #2481 from infosiftr/deprecate-mono

Deprecate `mono` image (due to maintainer inactivity)

mono/README-short.txt

@@ -1 +1 @@
-Mono is an open source implementation of Microsoft's .NET Framework
+DEPRECATED; Mono is an open source implementation of Microsoft's .NET Framework

mono/deprecated.md

@@ -0,0 +1 @@
+This image is deprecated due to maintainer inactivity (last updated Jun 2022; [docker-library/official-images#12682](https://github.com/docker-library/official-images/pull/12682)).