main/postmarketos-ui-plasma-mobile: switch to Dolphin from Index (MR 6255)

[ci:skip-vercheck]

Dolphin is now mobile friendly, we can use it instead of Index as the
default file manager.

main/postmarketos-ui-plasma-mobile/APKBUILD

@@ -16,7 +16,7 @@ depends="
 	"
 _pmb_recommends="
 	angelfish
-	index
+	dolphin
 	kclock
 	koko
 	krecorder