fix: don't use ancient legacy burst property with gatling shotgun

[chaosvolt]

Checklist

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• I wrote the PR title in conventional commit format.
• I ran the code formatter.
• I linked any relevant issues using github keyword syntax so it can be closed automatically.
• I have committed my changes to new branch that isn't main so it won't cause conflict when updating main branch later.

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Purpose of change

Smol easy fix pointed out in the discord.

Describe the solution

Converted gatling shotgun to use the fire mode JSON all other select-fire guns use instead of the old-as-fuck burst property.

Describe alternatives you've considered

Removing it from the code entirely and making modders cry.

Testing

Used my fucking eyes.

Additional context

[RoyalFox2140]

Now it's orphaned. Idk what we want to do about that. It appears only one Kenan pack mod uses it, idk about independent mods.

https://github.com/search?q=repo%3AZlorthishen%2FBrightNights-Structured-Kenan-Modpack%20%22burst%22%3A%20&type=code