Balance Idea: move the Seraphim ACU's advanced gun upgrade to the right arm

[Basilisk3]

Description of the proposed changes

This simple PR moves the Seraphim ACU's advanced gun upgrade to the right arm. The regular (speed) gun upgrade is now the prerequisite for upgrading the advanced (splash) gun.

Advantages of this approach

• The upgrade is underused due to competing with Tech 3 and Regen Aura, which are more versatile options. Both Tech 3 and Regen Aura are also cheaper, which means that by the time you can upgrade the advanced gun, you face the additional barrier of entry of having to remove two, very useful, upgrades.

Miscellaneous advantages

• Making it alongside the regular (speed) gun upgrade is basically a necessity anyway.
• The advanced gun upgrade alters the effectiveness of the gun, which is on the right arm, so it makes more sense for it to also be on the right arm.
• Having a unified and clear upgrade path is less confusing for new players who do not instinctively know which upgrade to get.
• Fewer misclicks.
• (Even number of upgrades (4) per slot).

Checklist

• [x] Changes are documented in the changelog for the next game version

[JaggedAppliance]

I think this is ok.

[Isocinetic]

I think there should be noticeable visual difference between just having gun and having gun+splash in order to allow intel counterplay