feat: add --update-only flag

[Gyan-max]

Add --update-only flag for database updates without scanning

This PR implements a solution for issue #4777 to improve the experience for users running multiple copies of cve-bin-tool in parallel.

Changes:

• Added a new --update-only flag that allows users to update the database without requiring a directory or file to scan
• When this flag is specified, the tool will:
• Update the database (with -u now behavior)
• Exit after the update is complete without proceeding to scanning
• Updated the documentation in:
• doc/MANUAL.md to include the new flag in the Database Management section
• doc/how_to_guides/multiple_scans_at_once.md to recommend using this flag in parallel usage scenarios
• Added a test case to verify the functionality

This addresses the first part of issue #4777, making it more intuitive for users to separate database updates from scanning when running multiple instances of the tool in parallel.