Show relevant errors when failing to match a CI job entry

What does this MR do?

CI jobs were identified only by the script keyword, resulting in unhelpful error messages:

build:
  before_script: pwd

Would raise this validation error: This GitLab CI configuration is invalid: root config contains unknown keys: build

This patch uses all the possible keys to match a job entry, resulting in this error message: "jobs:build script can't be blank".

Examples

• https://gitlab.com/guided-explorations/rough-drafts/multi-app-monorepo/-/blob/22d01b6f7b89cde4d2750539c201bb32c7fd604d/.gitlab-ci.yml
• https://gitlab.com/guided-explorations/rough-drafts/multi-app-monorepo/-/pipelines/164913721

Screenshots

What doesn't this MR do

It doesn't fix the error message for combining bridge jobs with regular jobs. Using a script with trigger in the same job is still returning jobs:build config contains unknown keys: trigger

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team