That message usually means that the private key you provided as the Jenkins credential named "The Jenkins user ssh private key" is not being accepted by the git repository at ssh://firstname.lastname@example.org:29418/project. Command line git invokes ssh using the ssh private key from that credential. If ssh authentication does not succeed, then ssh defaults to prompting the user to enter username and password. After 10 minutes, that prompting times out.
It is rare that these types of issues are bugs. They are most often configuration errors because a public key was not registered on the remote repository server or because a private key type (ED25519, or others) is not supported by the remote repository server (some older CentOS machines did not support the same ssh authentication schemes as newer operating systems), or because the key is passphrase protected and the wrong passphrase was provided, or because the git version being used on the agent is so old that it is not supported by the Jenkins git plugin (for example, CentOS 6 defaults to deliver CLI git 1.7 and that is too old for official support by the Jenkins git plugin).
Please use the mailing lists and the chat system for this type of configuration question. That allows many more people to assist with the issue instead of the very few people who monitor the plugin bug reports. If a discussion on the mailing list or in the chat systems decides that it is a bug, you can reopen this bug report or create a new bug report.