engine/install: remove Ubuntu 25.04 ("plucky"), which is EOL

Ubuntu 25.04 ("plucky") reached EOL on January 15, 2026;
https://documentation.ubuntu.com/project/release-team/list-of-releases/#end-of-life

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Author: thaJeztah

content/manuals/engine/install/ubuntu.md

@@ -52,7 +52,6 @@ To install Docker Engine, you need the 64-bit version of one of these Ubuntu
 versions:
 
 - Ubuntu Questing 25.10
-- Ubuntu Plucky 25.04
 - Ubuntu Noble 24.04 (LTS)
 - Ubuntu Jammy 22.04 (LTS)