Retire Unit. (#2630)

Refs: https://github.com/docker-library/official-images/issues/20039

unit/README-short.txt

@@ -1 +1 @@
-Official build of NGINX Unit: Universal Web App Server
+DEPRECATED; Official build of NGINX Unit: Universal Web App Server

unit/deprecated.md

@@ -0,0 +1,3 @@
+As of October 2025, NGINX Unit is archived and unmaintained. The images will not receive release or security updates.
+
+https://github.com/nginx/unit?tab=readme-ov-file#note-this-repository-has-been-archived-there-will-likely-be-no-further-development-at-this-repo-and-security-vulnerabilities-may-be-unaddressed