[BUGFIX] Apply correct default classes to `GenericButton`

The class `TYPO3\CMS\Backend\Template\Components\Buttons\GenericButton`
accidentally applied the wrong CSS classes on the rendered button.
This bugfix now applies the correct classes for buttons and provides
setter and getter methods for classes.

Resolves: #99841
Related: #99806
Releases: main
Change-Id: I3125a1208481da3d488b50847978171ca0b1f2bb
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77710


Tested-by: Benjamin Kott <benjamin.kott@outlook.com>
Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benjamin Kott <benjamin.kott@outlook.com>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>