Update CHANGELOG for 2.4 release (opensearch-project#5067)

There are a few changes in this commit:
 - For new features (e.g. Point in Time Search), I've replaced all the
   incremental commits with a single line pointing to the main GitHub
   issue describing the feature.
 - The 2.x section was mostly wrong with either duplicates or incorrect
   entries. I've consolidated it into the main 2.4 section (which was
   previously labeled as 'unreleased')
 - All changes related to flaky test fixes, GitHub workflows, etc have
   been removed as they are not useful to users reading a CHANGELOG.
 - I've moved all changes in CHANGELOG to a 2.4 section and created a
   new "Unreleased 2.x" section.

Signed-off-by: Andrew Ross <[email protected]>