Fix Dependabot security group schema

[aduth]

🛠 Summary of changes

Continues to attempt to fix Dependabot configuration issues introduced in #10576:

The property '#/updates/0/groups/security' of type object did not match one or more of the required schemas
The property '#/updates/1/groups/security' of type object did not match one or more of the required schemas

https://github.com/18F/identity-idp/runs/24825325838

Reference documentation: https://docs.github.com/en/code-security/dependabot/dependabot-version-updates/configuration-options-for-the-dependabot.yml-file#groups

You must use the patterns, exclude-patterns, dependency-type, or update-types options to define the group, or any combination thereof.

📜 Testing Plan

As before, it's not possible to test prior to merge (see dependabot/dependabot-core#4605).

At some point, I may abandon this and revert back to the configuration prior to #10576 if issues continue.

[aduth]

Strangely, this pull request has a Dependabot check that I don't recall seeing in previous pull requests 🤔 At least this might be a good sign that the configuration is valid now?