net-im/linux-discord: Downgrade version to 0.0.89

• The developers of Discord have identified an issue impacting voice sound on 0.0.90 version and have rolled back. (See https://discordstatus.com/incidents/mv9k3px764kd )
• Bump PORTEPOCH
• Add pkg-message to explain to users how to avoid automatic check for updates when starting the program.