Add security developer workflow template

What does this MR do?

We are moving to a new process for including security issues in planned security releases. See gitlab-com/gl-infra/delivery#19686 (closed) for full details on the new process.

This MR adds a new issue template for security developer workflow. Please let me know if this should be updated as most of the contents is copy/pasted from the matching Gitaly and Omnibus issue templates.

The new process adds a step after the issue development is complete to add the security-target label so the issue can be automatically linked when ready.

We also apply the security-notifications issue to the template. This allows the issue to receive automatic comments when the new stable branch has been created, which unblocks the last backport to be created.

We will be going through any currently in progress issues to makes sure they get these updates.

Related to gitlab-com/gl-infra/delivery#19680 (closed)

Update 2 files

• /.gitlab/issue_templates/Security developer workflow.md
• /PROCESS.md

TODO

• Feature flag
  • Added feature flag:
  • This feature does not require a feature flag
• I added the Changelog trailer to the commits that need to be included in the changelog (e.g. Changelog: added)
• I added unit tests or they are not required
• I added acceptance tests or they are not required
• I added documentation (or it's not required)
• I followed code review guidelines
• I followed Go Style guidelines