Update HISTORY.md for #8428 (#9001)

Update HISTORY.md for #8428 (#9001)

Summary:
Context:
HISTORY.md was not properly updated along with the change in https://github.com/facebook/rocksdb/pull/8428, where we introduced a change of accounting compression dictionary buffering memory and an extra condition of triggering data unbuffering.
Updated HISTORY.md for https://github.com/facebook/rocksdb/pull/8428 in 6.25.0 HISTORY.md section.
Updated blog post https://rocksdb.org/blog/2021/05/31/dictionary-compression.html.

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9001

Reviewed By: ajkr

Differential Revision: D31517836

Pulled By: hx235

fbshipit-source-id: 01f6b30de4e1ff6b315aa8221139d9b700c7c629