Fix Loading Icon on Instance-Level Security Dashboard Is Clickable

What does this MR do?

This resolves #241763 (closed) by fixing that the loading icon on the instance-level security dashboard is clickable when loading more vulnerabilities. The MR also adds the text Load more vulnerabilities to the button (as @andyvolpe said here: #241763 (comment 404840765)). 😄

This makes the instance-level security dashboard loading button behave like the button on the project-level dashboard.

Screenshots

Before

After

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

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Closes #241763 (closed)