feat(balance): implement armor_multiplier for several shotgun-type items lacking it

[chaosvolt]

Purpose of change (The Why)

This fixes some damage rebalancing that got lost along the way when AoE shotshells got tweaked a while back, and adds some other consistency updates as well.

Describe the solution (The How)

1. Added an armor multiplier of 2 to 20x66mm shotshells, with slugs and flechettes updated accordingly. This one I didn't see being removed by revert: bring aoe shotshell again #4950 so I suspect it might just be a case where it got overlooked back when shotshells started getting armor multipliers added to them.
2. Added the expected armor mults to all 40mm buckshot. Again, I dunno whether that just got yoinked by an even earlier PR or if it just got overlooked to begin with.
3. Added armor mult to .410 shot.
4. Restored flintlock shot's damage back to a rounded result of 60 damage with the standard shot armor mult.
5. Fixed blunderbuss armor mults having somehow got done up weirdly such that slugs had worse armor multiplier than shot.
6. We also missed ratshot entirely, oops.
7. And added to our much-underloved shotcanisters too, varied them up a bit since their shtick seems to mainly be varying types of shot.

Describe alternatives you've considered

Try to remember how to PR into an existing PR and offer them up as direct changes in #6034

Testing

1. Checked affected files for syntax and lint errors.
2. Load-tested in compiled test build.
3. Confirmed .22 ratshot still has the same listed damage as CPHP but the 3x armor multiplier typical of birdshot.
4. Confirmed 20mm flechette correctly doesn't mention armor mult, meaning it's 1x.
5. Checked a few other ammo items and didn't spot any oddities.

Additional context

Checklist

Mandatory

• I wrote the PR title in conventional commit format.
• I ran the code formatter.
• I linked any relevant issues using github keyword syntax like closes #1234 in Summary of the PR so it can be closed automatically.
• I've committed my changes to new branch that isn't main so it won't cause conflict when updating main branch later.

[RoyalFox2140]

Under the assumption that "armor_multiplier": -1 means that it's resetting flechette back to standard arpen (Because its copying from buckshot) This is fine

[chaosvolt]

yee