[submodule] Advance sonic-gnmi pointer

Update sonic-gnmi submodule pointer to include the following:
* 99bfa8f Remove LOGLEVEL DB since is no longer used ([sonic-net#56](sonic-net/sonic-gnmi#56))
* 6b0253a Add conditional check for split ([sonic-net#55](sonic-net/sonic-gnmi#55))
* ae72767 Add gnmi_dump tool for debug and unit test ([sonic-net#60](sonic-net/sonic-gnmi#60))
* 8226e46 Upgrade pipeline to use bullseye. ([sonic-net#58](sonic-net/sonic-gnmi#58))

Signed-off-by: dprital <drorp@nvidia.com>