Update Version number to 5.0.0

* configure.ac: Bump version to 5.0.0

This is helpful to distinguish upcoming release lines:
GnuPG 2.5/2.6 based releases with KF6 (64 bit) -> Gpg4win-5.x
GnuPG 2.4 based with kf5 -> Gpg4win-4.x
GnuPG 2.2 based -> Gpg4win 3.2.x