scripts: Update minimum required toolchain for the v2.6.0 release

'2.6.20240304' is pointer to 'v2.6.0-rc2' in toolchain-manager index
This is required by CMAKE_VERSION.
https://cmake.org/cmake/help/latest/variable/CMAKE_VERSION.html

Signed-off-by: Thomas Stilwell <Thomas.Stilwell@nordicsemi.no>

Author: thst-nordic

scripts/ncs-toolchain-version-minimum.txt

@@ -1,3 +1,3 @@
 # This file specifies the minimum nRF Connect SDK Toolchain that will be
 # working with this release.
-nrf-connect-sdk-toolchain=2.5.20231017
+nrf-connect-sdk-toolchain=2.6.20240304