Upgrades all the buttons in the admin dir

What does this MR do?

This MR uses the following Regex to add the appropriate gitlab ui button classes to a good chunk of the old buttons in the app/views/admin/ directory (excluding the application settings as this is handled in !44989 (merged)):

File	Find	Replace
app/views/admin/**/*.haml	(^(?!.*gl-button).*class:.*)btn (.*$)	$1gl-button btn $2

• Example: class: "btn btn-success" => class: "gl-button btn btn-success"
• Regex 101: https://regex101.com/r/8fawWo/1/

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

Note: This has not been manually tested for every button. It's a low-impact change and there's another few hundred to get through. Checking them all would take forever.

• 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