doc: software_maturity: H20 and LM20 update

remove encrytpion for LM20
add immutable MCUBoot for H20

Signed-off-by: Mateusz Michalek <[email protected]>

Author: michalek-no

doc/nrf/releases_and_maturity/software_maturity.rst

@@ -2794,7 +2794,7 @@ The following table indicates the software maturity levels of the support for ea
               - nRF54L15
               - nRF54LM20
             * - **Immutable MCUboot as part of build**
-              - --
+              - Experimental
               - Supported
               - Supported
               - Supported
@@ -2828,7 +2828,7 @@ The following table indicates the software maturity levels of the support for ea
               - Experimental
               - Experimental
               - Experimental
-              - Experimental
+              - --
 
       .. tab:: nRF91 Series