Update changelog on main (opensearch-project#5092)

This branch copies the release notes from 2.4 and removes all the
redundant entries from the changelog on main. All the remaining entries
in the changelog should reflect changes intended for 3.0.

This also adds an FAQ detailing how the changelog should work with our
branching strategy.

Signed-off-by: Andrew Ross <andrross@amazon.com>

Signed-off-by: Andrew Ross <andrross@amazon.com>

Author: andrross