[assumeutxo] add release notes

Summary:
Now that loadtxoutset is usable with a snapshot at block 896'800, let's add a release note.

Depends on D18093

Test Plan: proof-reading

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18099