Update socket2 requirement from 0.4 to 0.5 in /backend

[dependabot[bot]]

Updates the requirements on socket2 to permit the latest version.

Changelog

Sourced from socket2's changelog.

0.5.1

Added

• Type::cloexec for Redox and Solaris (rust-lang/socket2#398).
• Generate documentation for more targets on docs.rs (rust-lang/socket2#398).

Fixed

• Generatation of documentation on docs.rs (rust-lang/socket2#398).

0.5.0

Changed

• BREAKING SockAddr::init is renamed to try_init to indicate it can fail (rust-lang/socket2#328).
• BREAKING Remove the returned Result from SockAddr::vsock, it can't fail (rust-lang/socket2#328).
• BREAKING From<S> is now implemented using the I/O traits AsFd and AsRawSocket (rust-lang/socket2#325):
• BREAKING renamed SockAddr::vsock_addr SockAddr::as_vsock_addr to match the IPv4 and IPv6 methods (rust-lang/socket2#334).
• Redox now works on a stable compiler (rust-lang/socket2#326).
• Remove copy from From<SocketAddrV{4,6}> implementation for SockAddr (rust-lang/socket2#335).
• Marked function as constant where possible.
• Updated to Rust edition 2021 (rust-lang/socket2#393).

Added

• Links to OS documentation to a lot of methods (rust-lang/socket2#319).
• I/O-safety traits (rust-lang/socket2#325):
  • AsFd for Socket (Unix only).
  • From<OwnedFd> for Socket (Unix only).
  • From<Socket> for OwnedFd (Unix only).
  • AsSocket for Socket (Windows only).
  • From<OwnedSocket> for Socket (Windows only).
  • From<Socket> for OwnedSocket (Windows only).
• Unix socket support on Windows (rust-lang/socket2#249).

... (truncated)

Commits

• 621aba3 Release v0.5.1
• aebcbc4 Split checking of docs in CI
• b632eb4 Allow some broken links
• bf8f5ba Add Type::cloexec for Redox
• 040f826 Add Type::cloexec for Solaris
• f111f9d Use dtolnay/rust-toolchain to install toolchain in CI
• 2dc7235 Fix various doc attributes
• 4b5728c Check docs in Check job
• a04b140 Document all targets on docs.rs
• 0ffbf0f Fix various doc links
• Additional commits viewable in compare view

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