net/rustconn: Update to 0.15.12

ChangeLog:

• https://github.com/totoshko88/RustConn/releases/tag/v0.15.5
• https://github.com/totoshko88/RustConn/releases/tag/v0.15.6
• https://github.com/totoshko88/RustConn/releases/tag/v0.15.7
• https://github.com/totoshko88/RustConn/releases/tag/v0.15.8
• https://github.com/totoshko88/RustConn/releases/tag/v0.15.9
• https://github.com/totoshko88/RustConn/releases/tag/v0.15.10
• https://github.com/totoshko88/RustConn/releases/tag/v0.15.11
• https://github.com/totoshko88/RustConn/releases/tag/v0.15.12

Reported by: portscout!