Resolve "Resolve cross DB issues in ee/app/services/vulnerabilities/security_finding/create_issue_service.rb"

What does this MR do and why?

Update SecurityFinding::CreateIssueService.execute to resolve cross-DB modification

The method creates Vulnerability, Issue, and IssueLinks which are now in separate DB
and can therefore not be queried in the same transaction
To solve this the transaction has been removed and instead, we handle the rollback exception explicitly by destroying the changes made

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Related to #480894 (closed)