Update Security with new tools

Update the Security part to use GitLab trackers instead of Bugzilla. At the same time, clarify the process from the side of the project team. Big lines:

• Use GitLab for general vulnerability reporting
• Use GitLab for CVE assignements
• Allow vulnerability handling with GitHub Security Advisories
• Make it clear that a release with the bug fix is needed, not just a fix

Signed-off-by: Marta Rybczynska marta.rybczynska@eclipse-foundation.org