[JENKINS:SECURITY-440] Arbitrary file read vulnerability in SSH Credentials Plugin with Credentials Binding Plugin

SSH Credentials Plugin allowed the creation of SSH credentials with keys "From a file on Jenkins controller".
Credentials Binding Plugin 1.13 and newer allows binding SSH credentials to environment variables.
In combination, these two features allow users with the permission to configure a job to read arbitrary files on the Jenkins controller by creating an SSH credential referencing an arbitrary file on the Jenkins controller, and binding it to an environment variable in a job.

SSH Credentials Plugin no longer supports SSH credentials from files on the Jenkins controller file system, neither user-specified file paths nor ~/.ssh.
Existing SSH credentials of these kinds are migrated to "directly entered" SSH credentials.

NOTE: If plugin:blueocean[Blue Ocean] is installed, it needs to be updated to 1.5.1 or 1.6.1, or the creation of pipelines for plain Git will not work anymore after installing the fix for this issue.