Update mimalloc submodule to v2.1.9 and set branch to master in .gitmodules#131

dertin wants to merge 1 commit intopurpleprotocol:masterpurpleprotocol/mimalloc_rust:masterfrom

dertin:update-mimalloc-submoduledertin/mimalloc_rust:update-mimalloc-submoduleCopy head branch name to clipboard