samples: net: sockets: net_mgmt: Update flash requirement for twister

The net_mgmt sample enables a lot of features, resulting in pretty large
image sizes for various platforms (~200k). At the same time, the
sample.yaml for the sample did not specify minimum flash requirement for
the sample, causing flash overflows during build in certain cases.
This commit fixes this by setting a reasonable flash requirement for the
sample.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>