Fix return portal position calculation for dragon respawn

[OpenBagTwo]

Fixes #327 by explicitly flooring removing the (now)-unnecessary translations applied to the X and Z coordinates of this.portalLocation

Validation Performed

• Successfully respawned the dragon in an existing BetterX world (that I think was originally created in Minecraft 1.18)
• Successfully respawned the dragon in a brand new BetterX world created in Minecraft 1.20.4 with BCLib 3.30.1 and a build of this branch of BetterEnd

In both cases, the log message read:

Assuming portal is centered at (0.0, 0.0, 0.0)

though, so if there are scenarios where the portal could be elsewhere, I haven't confirmed that to be working.

Also note that I'm only checking from y=0 to y=255 (rather than -255 to 255) because I don't think there's any scenario in which the portal will generate at a negative y level (if I'm wrong, feel free to change back!)

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Stepping back, I realize that this actually could have been a two-line change to just remove the +/- 1 from

BetterEnd/src/main/java/org/betterx/betterend/mixin/common/EndDragonFightMixin.java

Lines 77 to 78 in 35c56c5

	new Vec3(central.getX() - 1, central.getY() - 255, central.getZ() + 1),
	new Vec3(central.getX() - 1, central.getY() + 255, central.getZ() + 1)

so feel free to just do that instead and close this PR.

[OpenBagTwo]

Looks like this is fixed on 1.21, but I would still highly recommend you merge this into the 1.20.3 branch.