Remove UNRELEASED modifier in changelog

I'm going to stop using those. They are redundant, because when
something is released, you can tell. Also, unlike some, we don't
auto-generate change entries with every commit, but just add them at the
end. This is extra note is more trouble than it is worth.

Signed-off-by: mulhern <[email protected]>

Author: mulkieran

CHANGES.txt

@@ -1,5 +1,5 @@
-libcryptsetup-rs 0.12.0 (UNRELEASED)
-====================================
+libcryptsetup-rs 0.12.0
+=======================
 Recommended Rust toolchain version: 1.84.0
 Recommended development environment: Fedora 40