Bluetooth: Mesh: Fix missing implementation of Proxy SAR timeout

Mesh Profile 1.0 Section 6.6:
"The timeout for the SAR transfer is 20 seconds. When the timeout
expires, the Proxy Server shall disconnect."

This will let qualification test case MESH/SR/PROX/BV-05-C pass
without requiring an explicit disconnect.

Fixes #16600

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>