Fix deprecated class finder script

What does this MR do and why?

Follow-up from !130 (comment 2000118043)

Bug fix for the deprecated class finder script. We need to ensure an exact match, instead of a partial match.

This fixes an issue with the previous version that made incorrect replacements like these:

gl-spinner-container => gl-animate-spinner-container
gl-spinner => gl-animate-spinner
gl-spinner-light => gl-animate-spinner-light
gl-spinner-md =>  gl-animate-spinner-md

We only want to replace gl-spin if it's a standalone class name, not a fragment of another class name.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/technical-writing-group/gitlab-docs-hugo/-/blob/main/doc/setup.md.
2. Run make lint-frontend. It should not find any deprecated classes.
3. Add a gl-spin class somewhere, and run make lint-frontend again. This class should raise a warning.

Merge request acceptance checklist

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