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]

Autofix has formatted code style violation in this PR.

I edit commits locally (e.g: git, github desktop) and want to keep autofix

1. Run git pull. this will merge the automated commit into your local copy of the PR branch.
2. Continue working.

I do not want the automated commit

1. Format your code locally, then commit it.
2. Run git push --force to force push your branch. This will overwrite the automated commit on remote with your local one.
3. Continue working.

If you don't do this, your following commits will be based on the old commit, and cause MERGE CONFLICT.

[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