Use Path::Tiny instead of File::Slurp in tools/clean-old and tools/dmg2mar

Similarly to legacy/trac#24361 (moved) and legacy/trac#28771 (moved), we should replace uses of File::Slurp by Path::Tiny in tools/clean-old and tools/dmg2mar.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information