Change recipient type for identity encryption to p256tag

Encrypting to an identity requires the plugin binary, and there is a
reasonable expectation that the same (or a later) plugin binary version
will be used to decrypt, so we can assume support for the preferred
recipient type.

CHANGELOG.md

@@ -16,6 +16,8 @@ to 0.3.0 are beta releases.
 
 ### Changed
 - MSRV is now 1.70.0.
+- Encryption to an identity now uses the preferred recipient type supported for
+  that identity.
 
 ## [0.5.0] - 2024-08-04
 ### Fixed