doc: release notes: add missing SoC, Board info in the 2.4.0 notes

Add some missing information regarding some nRF
SoC and Boards that were added in the current release.
Make a note that HW Stack protection is now enabled
by default in Nordic-maintained nRF Boards.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>