Ruby 2: Ensure YAML.safe_load_file exists

What does this MR do and why?

We are using YAML.safe_load_file in some places, but it only exists on Ruby 3. This currently fails Ruby 2 pipelines, e.g. https://gitlab.com/gitlab-org/gitlab/-/jobs/3861054630

Patch the YAML module for now until we are actually on Ruby 3, which should happen in the following weeks.

The same module leading to this failure was also mutating interpolated strings, which is not allowed in Ruby 2 if frozen-string-literals is set.

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