Update flyoutbutton class

[FiestaRuless]

PR Checklist

This PR fulfills the following requirements:

• The commit message follows our guidelines: Contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• A changelog entry was added

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• angular.io application / infrastructure changes
• Other... Please describe:

What is the current behavior?

Antwerp UI core update changed .a-button-outline to .a-button.a-button--outlined.
Needed to be updated for Flyoutbutton

Issue Number: N/A

What is the new behavior?

Correct styling

Does this PR introduce a breaking change?

• Yes
• No

Other information

Resolved issues

[FiestaRuless]

@TriangleJuice Ik kan je niet zelf als reviewer toevoegen, dus bij deze.