Replace `-depth` -> `-maxdepth` in boot_dev (PR #14046)

The -depth flag is incorrect on Linux, it does not take an argument and causes an error and results in no plugins ever being found.

Copied from man find:

The global options occur after the list of start points, and so are not the same kind of option as -L, for example.

       -d     A synonym for -depth, for compatibility with FreeBSD, NetBSD, MacOS X and OpenBSD.

       -depth Process each directory's contents before the directory itself.  The -delete action also implies -depth.


       -maxdepth levels
              Descend at most levels (a non-negative integer) levels of directories below the starting-points.  Using -maxdepth 0  means
              only apply the tests and actions to the starting-points themselves.


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This looks good to me. Thank you @jacobmischka for your contributions to Discourse :+1: