2.5 squad marines per 1 larva latejoin (cmss13-devs#4021)

# About the pull request

This PR lowers the latejoin barrier from 3 to 2.5 for weighted marine
joins.

Every 2.5 squad marines gives 1 larva.

# Explain why it's good for the game

larva time

# Testing Photographs and Procedure
<details>
<summary>Screenshots & Videos</summary>

Put screenshots and videos here with an empty line between the
screenshots and the `<details>` tags.

</details>


# Changelog

:cl: Morrow
balance: 2.5 squad marines per 1 larva latejoin
/:cl:

code/__DEFINES/mode.dm

@@ -74,7 +74,7 @@
 #define ROUNDSTATUS_FOG_DOWN 1
 #define ROUNDSTATUS_PODDOORS_OPEN 2
 
-#define LATEJOIN_MARINES_PER_LATEJOIN_LARVA 3
+#define LATEJOIN_MARINES_PER_LATEJOIN_LARVA 2.5
 
 //=================================================
 #define SHOW_ITEM_ANIMATIONS_NONE 0 //Do not show any item pickup animations