chore: Create Security Policy

[tomarra]

Description

In an attempt to make the OSV Scanner project a bit more visible in the Dart/ Flutter ecosystem we should be adding a Security policy to our templates as well as the repo overall. This will allow for people to be able to report issues to us as well as give us the ability to mark older versions as not supported.

Requirements

• No functional changes to the code
• We have an overall SECURITY.md file in the repo
• For each of the templates we have a SECURITY.md file filled out as a generic template so it is included in codebase generations from the start.