feat: add scanning of `/etc/os-release` for RHEL-based images

[DmitriyLewen]

Description

While Trivy currently relies on package metadata and release files like /etc/redhat-release, these may not always be present—especially in minimal or containerized environments (e.g. Calico images)
The os-release file is the standard file, so parsing this file will help avoid missing OS detection.

Required changes

• Update release.go file
  • add RHEL-based images into switch
• Add tests for new OSes
  • update test cases

[teddygood]

Hi, I'd like to work on this issue. Could you please assign it to me?