Handle YAML parsing error in SAST config mutation

What does this MR do and why?

Gracefully handles a YAML parsing error possible while modifying the CI config to add Secure feature specific snippet.

The exception handling is done for the following Secure features:

• Secret Detection
• SAST
• Dependency Scanning
• Container Scanning

Relevant issue: #386852 (closed)

Screenshots or screen recordings

Secret Detection

SAST

Dependency Scanning

Container Scanning

How to set up and validate locally

1. Create a new repository and add some files to it.
2. Within the repository create a .gitlab-ci.yml. Specifically, add at least one anchor/alias to it. Check below for an example.
3. Go to "Security and Compliance" > "Configuration". Choose "Enable SAST" and on the next screen, "Create merge request"
4. Doing so will lead to an error visible in the network logs. The HTTP status will be 200.

Example .gitlab-ci.yml with anchor and alias

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.