Declare version with "Version" rather than "Package-Version"

Emacs builtin packages are supposed to specify their version using the header
"Version" for their version to be correctly stored in the variable
package--builtins.

This is because the function finder-compile-keywords, responsible for setting
this variable, parses the source files looking for this exact header.

This does not break package, since the function package-buffer-info looks for
both Package-Version and Version.

lisp/transient.el

@@ -6,7 +6,7 @@
 ;; Homepage: https://github.com/magit/transient
 ;; Keywords: extensions
 
-;; Package-Version: 0.4.3
+;; Version: 0.4.3
 ;; Package-Requires: ((emacs "26.1") (compat "29.1.4.1") (seq "2.24"))
 
 ;; SPDX-License-Identifier: GPL-3.0-or-later