boards: nucleo_wba55cg: Update blob fetch command

Following addition of a `name` to hal_stm32 module, the command to fetch
blobs that are required to build BLE applications on STM32WBA socs is now
changed from `west blob fetch stm32` to `west blob fetch hal_stm32`.
Update documentation and warn users in migration guide.

Signed-off-by: Erwan Gouriou <erwan.gouriou@st.com>