Replace deprecated buttons on board view

What does this MR do?

Remove deprecated button classes from:

• Within the Edit Board modal, the “Save changes” button
• The “Load more issues” button at the bottom

Screenshots (strongly suggested)

No visual change

Does this MR meet the acceptance criteria?

Conformity

• 📋 Does this MR need a changelog?
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  • I have not included a changelog entry because _____.
• 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

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• 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