Stop using deprecated macOS API in fsmonitor.
* jh/fsmonitor-darwin-modernize:
fsmonitor: eliminate call to deprecated FSEventStream function
This backport to `maint-2.39` is needed to be able to build on
`macos-13`, which we need to update to as we restore the CI health of
that branch.
Signed-off-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>