Storybook 7 cleanup

What does this MR do?

This migrates some deprecated usages of Storybook's API that will be removed in Storybook 7:

• The disabled parameter has been renamed to disable: https://github.com/storybookjs/storybook/blob/next/MIGRATION.md#deprecated-disabled-parameter
• control.options has been deprecated in favor of defining the possible options at the argType level: https://github.com/storybookjs/storybook/blob/next/MIGRATION.md#deprecated-controloptions

Additionally, the size binding has been removed from GlButtonGroup's story as size isn't actually defined.

All affected stories have been smoke-tested and are working as expected after those changes.