Merge #622: Add changelog entry for removal of ONE_KEY

3b8a82e Add changelog entry for removal of ONE_KEY (Tobin C. Harding)

Pull request description:

  In the 0.25.0 release we removed the `ONE_KEY` const but did not mention it in the release notes. This makes upgrade harder than it should be.

  In hindsight we should have deprecated the const then removed it.

  Add a changelog entry to the 0.25.0 release mentioning the removal and the work around.

ACKs for top commit:
  apoelstra:
    ACK 3b8a82e

Tree-SHA512: 0bbf47a2f0e5221719b3d04a895c8f784623e31fd149911cf1b92afa2487037ee9619cedc94ea1d22bffd540ef5aa94a56e98daa56751c94274c973825ec9d5d

Author: apoelstra

CHANGELOG.md

@@ -23,6 +23,7 @@
 * [Introduce `rustfmt`](https://github.com/rust-bitcoin/rust-secp256k1/pull/499) to the codebase.
 * [Make all raw pointer methods go through the CPtr trait](https://github.com/rust-bitcoin/rust-secp256k1/pull/507).
 * [Make comparison functions stable](https://github.com/rust-bitcoin/rust-secp256k1/pull/518).
+* [Remove](https://github.com/rust-bitcoin/rust-secp256k1/pull/512) public constant `ONE_KEY` (consider using `FromStr` as a replacement).
 
 # 0.24.1 - 2022-10-25