Add experimental support for Spring Boot 4 and Spring Framework 7 in WebFlux

[sergiofdezfernandez]

What does this PR do?

This PR adds experimental support for Spring Boot 4 / Spring Framework 7 in the WebFlux instrumentation plugin by replacing all usages of the removed HttpHeaders.get(Object) API with HttpHeaders.getValuesAsList(String), which exists with the same signature across Spring Framework 5.x, 6.x, and 7.x.

Closes: Issue 4472

Background

In Spring Framework 6+, org.springframework.http.HttpHeaders no longer extends MultiValueMap<String, String> (and therefore no longer implements Map). The inherited Map.get(Object) method was removed in Spring 7. Our advice classes were compiled against Spring 5, so at runtime on Spring Boot 4 they threw:
java.lang.NoSuchMethodError: 'java.util.List org.springframework.http.HttpHeaders.get(java.lang.Object)'

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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