[BREAKING] Make open mr settings configuration more flexible and consistent

Makes open mr limit configuration more consistent.

Adds separate options for dependency and security vulnerability updates.

open-pull-requests-limit now accepts following:

• -1 will disable the limit
• 0 will disable mr creation
• positive value will set the limit

Same options can now be set for vulnerability updates via open-security-pull-requests-limit.

Closes https://gitlab.com/dependabot-gitlab/dependabot/-/issues/296