Fix the error when enabling security scans with a MR

What does this MR do and why?

This MR updates the Security::CiConfiguration::BaseCreateService to load the existing CI content using Gitlab::Ci::Config::Yaml.load!. as recommended in this comment.

This change resolves the issue that prevented projects with existing CI files containing the !reference tag from enabling security scans using the Security configuration Configure with a merge request feature.

It appears that this also enables projects with existing CI that contain YAML anchors to utilize the Configure with a merge request feature.

References

Related to: #368951

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Create a new group
2. Create a new project in the group
3. Add a file templates/.gitlab-ci-example1.yml with the content:

.check-variable-bash:
  script:
    - echo creating environment

4. Create another project in the group
5. Add a .gitlab-ci.yml file with the content:

include:
  - project: '<group/project-created-on-step-2>'
    file: '/templates/.gitlab-ci-example1.yml'

test:
  script:
    - !reference [.check-variable-bash, script]

6. Go to Secure > Security configuration
7. Go to the Dependency Scanning section
8. Click on Configure with a merge request
9. Verify that a new MR is created

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.