Align the default `gpg-passphrase` input value with the maven-gpg-plugin
default environment variable name (MAVEN_GPG_PASSPHRASE). With this default,
setup-java writes no extra GPG configuration to settings.xml and the plugin
reads the passphrase from MAVEN_GPG_PASSPHRASE out of the box.
Also document that reading the passphrase from an environment variable via
`gpg.passphraseEnvName` requires maven-gpg-plugin 3.2.0 or newer.
Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>
Copilot-Session: 7d0a510e-aebf-4ec4-a667-efeb3e4edeb1