Fix: Spring 7 Webflux NoSuchElementError on HttpHeaders#entrySet()

[mtomik]

What does this PR do?

Since the original PR is stuck for some time,
I created another one with the same commits + a bit better solution for the core of this issue.

Also when I was running these newer tests I found one more issue:

Spring7ServerFunctionalInstrumentationTest#dispatchError test was failing due to TracedSubscriber not removing the reference of the subscription on cancel() from contextMap. so the solution might be the CancellationAwareSubscription that is just the wrapper calling the discardIf() after cancel.

Is it possible that the issue was there even with older versions of reactor (from older Spring version), but now in that Spring 7 ( reactor 3.8.6 ) it was always failing.

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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