Close the logger's file descriptor once the process is complete.

[tirkarthi]

Initially I thought of accessing the filehandle through proc.log._logger._file where log is the BoundLogger but they are private and mypy doesn't like it. The other approach I thought was to store the filehandle as part of DagFileProcessorProcess itself. Thus whenever the processor is popped out after everything is complete the filehandle associated with the DagFileProcessorProcess is also closed. filehandle could be renamed to log_file_handle or something. I opened the PR for discussion to see if there are other approaches.

Closes #46048

Tests can be improved by asserting for close call but wanted to check first if there are any better approaches.

[tirkarthi]

Thanks @phanikumv and @jedcunningham for the review.

[ashb]

Coming back to this many months later, I now worry that there is a memory leak in the dag processsor service, since otherwise popping the processor object out of the dict should have removed all references to the open filehandle, meaning the GC would close the file for us?