Resolve "Add missing `dependent: :nullify` on User for `has_many: created_projects`"

What does this MR do and why?

See #351064 (comment 1142781154) for more details.

This MR adds the missing dependent: :nullify clause on User model, for the association has_many: :created_projects.

Decision to choose dependent: :nullify over ON DELETE SET NULL has been discussed here.

Without this, currently, whenever a User is destroyed, the user.id of this user still remains in the Project#creator_id column. And hence, project.creator always returns nil and further method calls on project.creator will fail with NoMethodError: undefined method for nil:NilClass

Once we add dependent: :nullify, such associations are nullified in batches, whenever a User is destroyed via the Users::DestroyService class, due to the code here.

We already have ~50k such orphaned Project records, so the next step is to nullify the creator_id column of these records using a post deploy migration: #378477 (closed), which will be done in another MR

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Related to #378476 (closed)