samples: net: echo_server: Decrease buffer for atsamr21_xpro

From 2.3 to 2.4 the net samples pair echo-server/client increase the
SRAM requirements. Since CI doesn't build all combinations, (even for
a release like 2.4.0-rc1) the problem was detected only now.  Decrease
buffers for atsamr21_xpro on both echo_server/client sample pair.

Fixes #28341.

Signed-off-by: Gerson Fernando Budke <nandojve@gmail.com>