BTU_TASK_STACK_SIZE too small for Polar H10 heart rate monitor

[FIXMBR]

Default value set in sdkconfig.esp32 is:
CONFIG_BT_BTU_TASK_STACK_SIZE=4096

When trying to get services of Polar H10 heart rate monitor, it causes a stack overflow. I detailed the issue here espressif/arduino-esp32#6063.

After setting it to 8192 the library works fine.

As H10 is one of the more popular hr monitors, and other people had the same problem (nkolban/esp32-snippets#837, stackoverflow) I think it's okay to increase it in the official lib.