Bluetooth: Keys: Fix BT_KEYS_OVERWRITE_OLDEST logic for BT_MAX_CONN > 1

When the Bluetooth stack is configured for CONFIG_BT_MAX_CONN > 1
the oldest key might currently be in use.  Fix the logic to ensure the
oldest key overwritten is from the set of keys currently not in use.

Fixes: https://github.com/zephyrproject-rtos/zephyr/issues/35999



Signed-off-by: Faisal Saleem <faisal.saleem@setec.com.au>
Signed-off-by: Nick Ward <nick.ward@setec.com.au>