Update linux.md

Author: michal-at-travisci

user/reference/linux.md

@@ -9,12 +9,12 @@ layout: en
 This page gives an overview of the different Ubuntu Linux distributions you can use as your CI environment.
 
 You can choose one of the following distributions:
-* [Ubuntu Jammy 22.04](/user/reference/jammy/)
-* [Ubuntu Focal 20.04](/user/reference/focal/)
-* [Ubuntu Bionic 18.04](/user/reference/bionic/)
-* [Ubuntu Xenial 16.04](/user/reference/xenial/) **default**
-* [Ubuntu Trusty 14.04](/user/reference/trusty/) 
-* [Ubuntu Precise 12.04](/user/reference/precise/)
+* [Ubuntu Jammy 22.04](/user/reference/jammy/)  **recommended**
+* [Ubuntu Focal 20.04](/user/reference/focal/)  **recommended**
+* [Ubuntu Bionic 18.04](/user/reference/bionic/) *deprecation warning, will be available for some time more*
+* [Ubuntu Xenial 16.04](/user/reference/xenial/) **default** *deprecation warning: by end of 2024*
+* [Ubuntu Trusty 14.04](/user/reference/trusty/) *deprecation warning: by end of 2024*
+* [Ubuntu Precise 12.04](/user/reference/precise/) *deprecated*
 
 ## Using Ubuntu Linux distributions