Rename the old main-menu music to make it clear that it's now available

as mood music.

data/campaigns/Sceptre_of_Fire/scenarios/1_A_Bargain_is_Struck.cfg

@@ -90,7 +90,7 @@
         [part]
            # wmllint: local spelling Ay
            story= _ "Ay, the Sceptre of Fire. The sceptre has a long, glorious, and fearful history. But I am not here to tell you how the jewel within it was brought over the ocean by Haldric the Great. Nor will I tell the story of Garard I, and the challenge he set, or that of Konrad, king of Wesnoth by virtue of the Sceptre."
-            music=main_menu.ogg
+            music=main_menu_new.ogg
         [/part]
         [part]
             story= _ "I am here to tell you of its making. Of its crafting, deep in the caverns of dwarfdom. And of the dwarves who made it; for they were great, they were. They are counted among the greatest of the heroes of ancient Knalga."

data/campaigns/The_Hammer_of_Thursagan/scenarios/11_The_Court_of_Karrag.cfg

@@ -21,7 +21,7 @@
 
     {SCENARIO_MUSIC "the_king_is_dead.ogg"}	# No story part
     {EXTRA_SCENARIO_MUSIC "revelation.ogg"}
-    {EXTRA_SCENARIO_MUSIC "main_menu.ogg"}
+    {EXTRA_SCENARIO_MUSIC "transience.ogg"}
 
     [side]
         type="Dwarvish Fighter"

data/campaigns/Two_Brothers/scenarios/1_Rooting_Out_A_Mage.cfg

@@ -60,7 +60,7 @@
     [/story]
 
     [music]
-        name=main_menu.ogg
+        name=transience.ogg
         ms_before=500
         append=no
     [/music]
@@ -116,7 +116,7 @@
         [/objectives]
         
         [music]
-            name=main_menu.ogg
+            name=transience.ogg
             ms_before=500
             append=no
         [/music]

data/core/macros/sound-utils.cfg

@@ -35,7 +35,7 @@
         append=yes
     [/music]
     [music]
-        name=main_menu.ogg
+        name=transience.ogg
         ms_before=12000
         append=yes
     [/music]
@@ -136,7 +136,7 @@
             append=yes
         [/music]
         [music]
-            name=main_menu.ogg
+            name=transience.ogg
             ms_before=12000
             append=yes
         [/music]