fix(Scripts/Underbog): modernise boss scripts

[elthehablo]

Changes Proposed:

• use taskscheduler instead of events
• remove heroic mechanic on black stalker that swarms the group with adds

Issues Addressed:

• Closes

SOURCE:

Tests Performed:

How to Test the Changes:

Known Issues and TODO List:

• [ ] timers need to be looked at extensively
• [ ] muselek needs cpp script, but: https://github.com/azerothcore/azerothcore-wotlk/pull/14724

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For example: if a PR fixes spell X by changing a part of code that handles spells X, Y, and Z, we should not only test X, but we should test Y and Z as well.