Use in-memory PGP keys for Gradle signing

[ericlmao]

Replace useGpgCmd() and explicit mavenJava signing with in-memory PGP key signing in Gradle build files. Updated moss, moss-bungeecord, moss-paper, and moss-velocity to read signingKey and signingPassword project properties, call useInMemoryPgpKeys(signingKey, signingPassword), and sign publishing.publications so CI can sign artifacts without an external GPG agent.

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