net/rustconn: Update to 0.15.4

ChangeLog:

• https://github.com/totoshko88/RustConn/releases/tag/v0.15.1
• https://github.com/totoshko88/RustConn/releases/tag/v0.15.2
• https://github.com/totoshko88/RustConn/releases/tag/v0.15.3
• https://github.com/totoshko88/RustConn/releases/tag/v0.15.4

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