Merge #849: Add changelog for 0.31.1

ec8a6df Add changelog for 0.31.1 (Tobin C. Harding)

Pull request description:

  When we released the, albeit triival, point release we did not add a changelog. Do so.


ACKs for top commit:
  apoelstra:
    ACK ec8a6df; successfully ran local tests; nice


Tree-SHA512: 34f4040cac7f4be5e5287ed0559f992da0ff297875bc20e5990abdcb73d36db9e7bd90f7a34fdc4f99113b7ebbc29e87a7c5248064e9f313c7a219c1e8192f61

Author: apoelstra

CHANGELOG.md

@@ -1,3 +1,7 @@
+# 0.31.1 - 2025-06-23
+
+* Update deprecation notes with `since` instead of `TBD`.
+
 # 0.31.0 - 2025-04-21
 
 * Update `rand` to 0.9 [#788](https://github.com/rust-bitcoin/rust-secp256k1/pull/788)