Make user deactivation emails opt-in

What does this MR do?

Closes #337784 (closed)

Makes user deactivation emails opt-in via a new ApplicationSetting under admin > settings > preferences > email, restoring previous behavior prior to !64964 (merged)

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How to setup and validate locally (strongly suggested)

Does this MR meet the acceptance criteria?

Conformity

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• I have added/updated documentation, or it's not needed. (Is documentation required?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
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• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
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• This change is backwards compatible across updates, or this does not apply.

Availability and Testing

• I have added/updated tests following the Testing Guide, or it's not needed. (Consider all test levels. See the Test Planning Process.)
• I have tested this MR in all supported browsers, or it's not needed.
• I have informed the Infrastructure department of a default or new setting change per definition of done, or it's not needed.