MAINTAINERS: Update release notes owners for 4.0

As mentioned in the TSC Meeting of 7 August 2024
* Mahesh Mahadevan (mmahadevan108)
* Dan Kalowsky (dkalowsk)
will be the release managers for 4.0.

Let's update the MAINTAINERS files accordingly.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>