Remove beta from web terminal

closes gitlab-org/gitlab-ce#51295

doc/ci/interactive_web_terminal/index.md

 # Getting started with interactive web terminals
 
-> Introduced in GitLab 11.3.
-
-CAUTION: **Warning:**
-Interactive web terminals are in beta, so they might not work properly and
-lack features. For more information [follow issue #25990](https://gitlab.com/gitlab-org/gitlab-ce/issues/25990).
+> [Introduced](https://gitlab.com/gitlab-org/gitlab-ce/issues/50144) in GitLab 11.3.
 
 Interactive web terminals give the user access to a terminal in GitLab for
 running one-of commands for their CI pipeline.