samples: cellular: nrf_cloud_multi_service: More modem RX shared memory

The sample reads certificates while checking their validity
to give better error messages.
PR nrfconnect#17189 optimized modem shared memory to 4kB for both TX and RX.
This is too small for RX with several certificates.
Issues seen with three Root CAs causing modem to eventually crash
as there is not enough RPC memory for the reponse.
So increasing CONFIG_NRF_MODEM_LIB_SHMEM_RX_SIZE by letting it use
default value of ~8kB.

Jira: TNSW-64434

Signed-off-by: Tommi Rantanen <tommi.rantanen@nordicsemi.no>
(cherry picked from commit 1b9c394)

samples/cellular/nrf_cloud_multi_service/prj.conf

@@ -40,9 +40,9 @@ CONFIG_AT_HOST_STACK_SIZE=2048
 # Extended memory heap size needed both for PGPS and for encoding JSON-based nRF Cloud Device Messages.
 CONFIG_HEAP_MEM_POOL_SIZE=19000
 CONFIG_SYSTEM_WORKQUEUE_STACK_SIZE=4096
-# The default modem shared memory buffers are significantly larger than required for this sample
+# The default modem shared memory buffer for TX is significantly larger than required.
+# RX buffer has to be larger because the sample reads certificates so using default value.
 CONFIG_NRF_MODEM_LIB_SHMEM_TX_SIZE=4096
-CONFIG_NRF_MODEM_LIB_SHMEM_RX_SIZE=4096
 
 # Enable Networking and Connection Manager.
 CONFIG_NETWORKING=y