Fix GPG plugin usage

[zambrovski]

Steps to reproduce

• release new version

Expected behaviour

• No warnings in log

Actual behaviour

[INFO] --- gpg:3.2.5:sign (sign-artifacts) @ maven-parent ---
Warning:   Parameter 'passphrase' (user property 'gpg.passphrase') is deprecated: Do not use this configuration, it may leak sensitive information. Rely on gpg-agent or env variables instead.
Warning:  
Warning:  W A R N I N G
Warning:  
Warning:  Do not store passphrase in any file (disk or SCM repository),
Warning:  instead rely on GnuPG agent or provide passphrase in 
Warning:  MAVEN_GPG_PASSPHRASE environment variable for batch mode.
Warning:  
Warning:  Sensitive content loaded from Mojo configuration
Warning: