refactor(flag): return error if both --download-db-only and --download-java-db-only are specified

[DmitriyLewen]

Description

Trivy doesn't support simultaneous use of the --download-db-only and --download-java-db-only flags.
So in this case we need to return an error.

Example:

➜ trivy image --download-db-only --download-java-db-only
2024-07-30T12:19:15+06:00       FATAL   Fatal error     flag error: db flag error: --download-db-only and --download-java-db-only options can not be specified both

Related discussions

• --download-db-only and --download-java-db-only can't be used in together #7250

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