Update read receipt remainder for internal font size change
Update read receipt remainder for internal font size change

In https://github.com/matrix-org/matrix-react-sdk/pull/4725, we changed the
internal font size from 15 to 10, but the `toRem` function (currently only used
for read receipts remainders curiously) was not updated. This updates the
function, which restores the remainders.

Fixes https://github.com/vector-im/riot-web/issues/14127

matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk,matrix-org/matrix-react-sdk