Deprecate TARGET_FILE

Today ERT has a feature where a job can specify a TARGET_FILE, and ERT will then fail the job if that target file was not modified after the job was started, even if the exit code from the job was zero. However, this approach leads to ambiguous situations where we are not able to determine if the target file was actually changed, in particular when the modification time is the same as the start time (impossible to determine if the previous job touched the file or the current job did it). Relying on exit code is more robust than relying on last modification time of a file on disk, and if a job _must_ rely on checking whether a file is written or not it should do so in the job itself (or a wrapper script).

No files or tests are named in the issue. Start by searching the repository for TARGET_FILE and tracing its parsing, execution, and test coverage; confirm the intended deprecation boundary before changing anything. Done means the deprecated behavior is addressed consistently and the relevant tests reflect the change.

Written by the indexing model from the issue text.