Add deprecation notice for CentOS

centos/README-short.txt

@@ -1 +1 @@
-The official build of CentOS.
+DEPRECATED; The official build of CentOS.

centos/deprecated.md

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+This image is no longer supported/maintained (non-EOL tags last updated November 16, 2020, [docker-library/official-images#9102](https://github.com/docker-library/official-images/pull/9102); see also https://www.centos.org/centos-linux-eol/ and [docker-library/docs#2205](https://github.com/docker-library/docs/pull/2205)). Please adjust your usage accordingly.