licenses: approve mdi-react, pin license_finder

Created by: bobheadxi

closes https://github.com/sourcegraph/sourcegraph/issues/11866

mdi-react's license and version did not change, and its license was already on the allowlist, but for whatever reason the check no longer recognizes the license, so I've added an override for it:

license_finder approvals add mdi-react

I suspect it's a regression in license_finder, as there was an update recently. To hopefully prevent this from being an issue again I've pinned the version installed in both github actions