Revert "json: use latest to get gcc10 fixes"

This reverts commit 1976d396241155d7f8ffe443d2e49af6fb90d8cc.

nlohmann::json is now compatible.  This is a revert + changing the
nlohmann::json version to 3.9.1.

Signed-off-by: Ed Tanous <ed@tanous.net>
Change-Id: I0cf9c5ca30788fd137519079ef44297b4582c1fb
1 file changed