BLE_GattServer_AddService does not work on NRF52-DK

### Description of defect
This looks a lot like issue #339, which I'm not sure was resolved. I've updated here with any new details:

The BLE_GattServer_AddService example compiles ok. However when using a BLE scanner and then connecting to the "Heartrate" device it disconnects unexpectedly, immediately after establishing a connection.

I added a disconnect reason printout to the disconnect callback and the error is as was reported in the initial bug report (0x08, timeout).

The nrf connect log says:

`Connected.`
`Discovering Services...`
`peripheral.discoverServices(nil)`
`[Callback] centralManager(central, didDisconnectPeripheral: peripheral, error: The connection has timed out unexpectedly.)`

#### Target(s) affected by this defect ?
NRF52_DK

#### Toolchain(s) (name and version) displaying this defect ?
GCC_ARM

#### What version of Mbed-os are you using (tag or sha) ?

mbed-os-6.6.0

#### What version(s) of tools are you using. List all that apply (E.g. mbed-cli)
mbed-cli 1.10.4 (macintosh)
nrf connect 2.4.6 (iOS)
iPhone 11 (iOS 14.3)

#### How is this defect reproduced ?
compile BLE_GattServer_AddService with mbed-cli and drag resulting .hex file to the NRF52_DK Jlink connection. Use NRF Connect to connect with Heartrate device. Will disconnect immediately.

Start by reproducing BLE_GattServer_AddService on the NRF52_DK with mbed-os-6.6.0, GCC_ARM, mbed-cli 1.10.4, and nRF Connect, then inspect the connection and service-discovery behavior. Done means the Heartrate device stays connected while services are discovered instead of disconnecting with timeout reason 0x08.

Written by the indexing model from the issue text.