GitLab UI: rename GlButton to GlDeprecatedButton

• Related issue: gitlab-ui#608 (closed)
• Related MR: gitlab-ui!1223 (merged)

What does this MR do?

This is an integration branch to prepare the changes being made in this GitLab UI MR: gitlab-ui!1223 (merged)

To discourage the use of the old button component in favor of the new one. We're renaming GlButton to GlDeprecatedButton and GlNewButton will be renamed to GlButton.

In this first iteration, we're focusing on renaming GlButton to GlDeprecatedButton, which requires that we update all usages of GlButton in GitLab's codebase to rely on GlDeprecatedButton instead.

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