Docs: Update security policy

[vanessayuenn]

What I did

I removed the specific versions we support in SECURITY.md, and instead describe how we release security patches. This way, the policy does not get outdated as we release versions, and it accurately describes what we actually do in the event of a security fix.

How to test

Not necessary

Checklist

• Make sure your changes are tested (stories and/or unit, integration, or end-to-end tests)
• Make sure to add/update documentation regarding your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli/src/sandbox-templates.ts
• Make sure this PR contains one of the labels below.

["cleanup", "BREAKING CHANGE", "feature request", "bug", "build", "documentation", "maintenance", "dependencies", "other"]