Scripts/Ulduar: Exclude Ignis' main target from the Slag Pot target selector

Closes #10516

(cherry picked from commit f4c035554fdc25d286ddc37c24e3ab43c71f1dae)
Scripts/ScarletMonastery: Summon the Headless Horseman once the Call the Headless Horseman is quest complete

By @cr4st
Closes #13273

(cherry picked from commit 22f218540bedbe43fd5fefb049561dbeccddc6c5)
DB/Achievement: Fix map conditions for achievement "He's not getting any older"

25m used to be awarded in 10m vice-versa

(cherry picked from commit 805be8ebba07c3280089a128f1764d49f6f84ab6)
DB/World: Cherry-picks

- ee623a4006009e729ed4cfdd36aa9bb1525d72a9 & 3965ee33efd8751d64da6352227650fb4181fe31
- 8274caeef052ffaf21c9109de13b8c2a5355d9e4
- 805be8ebba07c3280089a128f1764d49f6f84ab6
- d680e82c38068d9f69393f9d32b3f10fc9729460