Log successful network disconnect at Info level

Pishelrequested to merge
gitlab-community/gitlab-org/gitlab-runner:fix/issue-39616-disconnect-network-log-level into main

What does this MR do?

Changes the log level for successful network disconnects from WARNING to Info, and removes the alarming "zombie container" wording.

Closes #39616 (closed)

Why?

Since commit 869d9e0e8 ("Fix disconnectNetwork container ID matching"), the disconnectNetwork() function actually works for the first time. However, the success branch was logging at WARNING level with "Possibly zombie container" wording, causing user confusion and support tickets.

A successful cleanup operation should not be logged as a warning.

Edited by Pishel

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