feat: add warning button variant

[FrediWa]

Description

Add the warning variant to Flow buttons.

Related PR: vaadin/web-components#7485

Type of change

• Bugfix
• Feature

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ FrediWa
❌ Fredrik Wasström

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[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud