[Enhancement] Switch to sign-maven-plugin

[Mechite]

sign-maven-plugin allows users on all platforms to build the project agnostically. It works the same as maven-gpg-plugin and will allow you to deploy the way you are doing so already.

This will also create the following diff for the README (after #1):

- 56   # Building
- 57   When building on a Windows platform (or anywhere where GPG is not installed, *nix systems tend to bundle GPG), you should pass the `-Dgpg.skip` argument to Maven (`mvnd clean install -Dgpg.skip`).

[Mechite]

Another note - Since GnuPG release 2.1.12, the default mode is –allow-loopback-pinentry, so you no longer need the following code (if you wish not to move to sign-maven-plugin and instead want to stay using maven-gpg-plugin):

- <arg>--pinentry-mode</arg>
- <arg>loopback</arg>