Remove not null constraint for confidence columns

What does this MR do and why?

This MR makes the confidence column optional for Vulnerability, Vulnerabilities::Finding, and Security::Finding models.

Related to Adjust security report models and ingestion for... (#370925 - closed).

Database review

rake db:migrate:up

main: == 20220825134827 RemoveNotNullConstraintForConfidenceColumns: migrating ======
main: -- change_column_null(:vulnerabilities, :confidence, true)
main:    -> 0.0012s
main: -- change_column_null(:vulnerability_occurrences, :confidence, true)
main:    -> 0.0005s
main: -- change_column_null(:security_findings, :confidence, true)
main:    -> 0.0004s
main: == 20220825134827 RemoveNotNullConstraintForConfidenceColumns: migrated (0.0024s)

rake db:migrate:down

main: == 20220825134827 RemoveNotNullConstraintForConfidenceColumns: reverting ======
main: -- change_column_null(:vulnerabilities, :confidence, false)
main:    -> 0.0014s
main: -- change_column_null(:vulnerability_occurrences, :confidence, false)
main:    -> 0.0007s
main: -- change_column_null(:security_findings, :confidence, false)
main:    -> 0.0006s
main: == 20220825134827 RemoveNotNullConstraintForConfidenceColumns: reverted (0.0033s)

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.