refactor: use redhat dataSource for redhat-oval

[DmitriyLewen]

Description

This PR consolidates the Red Hat OVAL data source to use the existing RedHat data source identifier instead of maintaining a separate RedHatOVAL identifier.

See more details in aquasecurity/trivy#9192 (comment)

Changes

• Data Source Consolidation: Changed redhat-oval vulnsrc to use vulnerability.RedHat instead of vulnerability.RedHatOVAL
• Updated Data Source Metadata:
  • ID: redhat-oval → redhat
  • Name: Red Hat OVAL v2 → Red Hat
  • URL: https://www.redhat.com/security/data/oval/v2/ → https://access.redhat.com/security/cve/
• Removed RedHatOVAL constant: Deleted the separate RedHatOVAL source ID from vulnerability/const.go and AllSourceIDs slice
• Updated Tests: Modified all test expectations in redhat-oval_test.go and test fixtures to reflect the new data source values

Related PRs

• fix(vuln): save package-specific severity before severity selection trivy#9192