correct release version

This was supposed to be 10.20260420.

Unfortunately, 10.20260421 made it to hackage so will necessarily exist
there. And as such, 10.20260421 is the version, so update CHANGELOG to
match the cabal file.

Will make a 10.20260421 tag from this. Will keep the 10.20260420 tag,
which corresponds to the tarball on cabal for 10.20260421.

Author: joeyh

CHANGELOG

@@ -1,4 +1,4 @@
-git-annex (10.20260420) upstream; urgency=medium
+git-annex (10.20260421) upstream; urgency=medium
 
   * disableremote: New command.
   * Fix annexUrl to inherit any password that is set in the remote url.