fix(webflux): fix NoSuchMethodError when using spring framework 7 wit…

[jihadbou15]

…h webflux

What does this PR do?

This PR fixes an issue with the experimental support for Spring framework 7 that was added PR. When running the agent, it will throw a NoSuchMethodError exception due to the HttpHeaders.entrySet method. This is due to HttpHeaders no longer implementing the MultiValueMap contract.
I have changed the copying of header to only use API's that are still available across version 5, 6 and 7.
The test co.elastic.apm.agent.springwebflux.AbstractServerInstrumentationTest#dispatchHello covers this fix.

Checklist

• This is an enhancement of existing features, or a new feature in existing plugins
  • I have updated CHANGELOG.next-release.md
  • I have added tests that prove my fix is effective or that my feature works
  • Added an API method or config option? Document in which version this will be introduced
  • I have made corresponding changes to the documentation
• This is a bugfix
  • I have updated CHANGELOG.next-release.md
  • I have added tests that would fail without this fix
• This is a new plugin
  • I have updated CHANGELOG.next-release.md
  • My code follows the style guidelines of this project
  • I have made corresponding changes to the documentation
  • I have added tests that prove my fix is effective or that my feature works
  • New and existing unit tests pass locally with my changes
  • I have updated supported-technologies.md
  • Added an API method or config option? Document in which version this will be introduced
  • Added an instrumentation plugin? Describe how you made sure that old, non-supported versions are not instrumented by accident.
• This is something else
  • I have updated CHANGELOG.next-release.md

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[jihadbou15]

Will remake the PR as i committed with the wrong account.