feat(balance): Alter "Clear Back Bay" mission so zombies don't spawn until mission is given

[Zlorthishen]

Purpose of change

This change will make the back bay zombies not spawn until the mission is requested, this will prevent zombies from breaking out before the player has a chance to accept the mission

Describe the solution

removes the zombie spawn from the map moves them to the NPC mission

Describe alternatives you've considered

continuing to allow the evolved zombies to kill all NPCs before I can speak to any of them

Testing

before requesting mission:

after requesting mission:

Additional context

Checklist

[autofix-ci[bot]]

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[chaosvolt]

Hmm. It'll definitely look weird but...I wonder, whether a doorway opening to some ruined stairs could be placed in that area so that it could imply that the zeds were hiding in part of the basement level, which is implied to exist but not currently present...that'd be more hassle, can probably fuck with that as a followup PR prolly