Replace deprecated buttons with GlButton in …modal_footer.vue

What does this MR do?

• Replaces the deprecated LoadingButton with GitLab UI’s GlButton.
• Replaces the deprecated GlDeprecatedButton with GitLab UI’s GlButton.

Screenshots

Location	Before	After
Go to CI/CD > Pipelines, select a pipeline, go to the Security tab, click on a vulnerability to open the modal. Note, you’ll need to have a local project with a runner and example vulnerabilities to test.

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

N/A

Security

N/A

Closes #229210 (closed), #219773 (closed)