doc: update versions for 2.4.0 release

Updated changelog versions for the NCS 2.4.0 release

Signed-off-by: Richa Pandey <[email protected]>

Author: richabp

mpsl/CHANGELOG.rst

@@ -7,9 +7,10 @@ Changelog
    :local:
    :depth: 2
 
-Main branch
-***********
-All the notable changes included in the main branch are documented in this section.
+nRF Connect SDK v2.4.0
+**********************
+
+All the notable changes included in the |NCS| v2.4.0 release are documented in this section.
 
 Changes
 =======

nrf_fuel_gauge/CHANGELOG.rst

@@ -9,7 +9,7 @@ Changelog
 
 All notable changes to this project are documented on this page.
 
-Main branch
-***********
+nRF Connect SDK v2.4.0
+**********************
 
 Initial release.

softdevice_controller/CHANGELOG.rst

@@ -9,10 +9,10 @@ Changelog
 
 All the notable changes to this project are documented on this page.
 
-Main branch
-***********
+nRF Connect SDK v2.4.0
+**********************
 
-All the notable changes included in the main branch are documented in this section.
+All the notable changes included in the |NCS| v2.4.0 release are documented in this section.
 
 Added
 =====