Update "flat" to v5.0.2

[ericyd]

Description of the change

Pulls in prototype pollution protection from activeprospect/flat#18

Type of change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Configuration change
• Technical Debt
• Documentation

Related tickets

https://app.shortcut.com/active-prospect/story/74086/submitting-lead-with-proto-property-crashes-handler

Checklists

Development and Testing

• Lint rules pass locally.
• The code changed/added as part of this pull request has been covered with tests, or this PR does not alter production code.
• All tests related to the changed code pass in development, or tests are not applicable.

Code Review

• This pull request has a descriptive title and information useful to a reviewer. There may be a screenshot or screencast attached.
• At least two engineers have been added as Reviewers on the pull request.
• Changes have been reviewed by at least two other engineers who did not write the code.
• This branch has been rebased off master to be current.

Tracking

• Issue from Shortcut/Jira has a link to this pull request.
• This PR has a link to the issue in Shortcut.

QA

• This branch has been deployed to staging and tested.

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❯ echo "sc-74086/submitting-lead-with-proto-property-crashes"
sc-74086/submitting-lead-with-proto-property-crashes