Spring6 webflux tests

[JonasKunz]

What does this PR do?

Part of #2942 .

• Adds the missing tests for webflux with spring 6
• Upgrades the webclient project to spring 6, added integration test to still also test with spring 5
• Simplified dependency management: We only use now the spring-boot dependency management, which should simplify dependabot upgrades and avoid incompatibilities between dependencies

I won't update the changelog for this PR, as it only adds testing.

Checklist

• This is something else
  • I have updated CHANGELOG.asciidoc

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• Start Time: 2023-04-27T14:39:18.322+0000

• Duration: 15 min 10 sec

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[jackshirazi]

nice clean code!