Replace kernel_arch with get_os_architecture

[zeroSteiner]

Now that #21418 has been merged, Metasploit has a platform agnostic way to obtain the OS/Kernel architecture. This technique has the added benefit of supporting meterpreter sessions directly (meaning we don't always run uname).
This PR removes the now redundant Linux-specific method so there's a single source of truth that works in a superset of platform / session-type combinations.

[Copilot]

Pull request overview

This PR removes the Linux-specific kernel_arch implementation and routes Linux kernel architecture detection through the platform-agnostic get_os_architecture, aiming for a single architecture source of truth that also supports meterpreter sessions without relying on uname.

Changes:

• Include Msf::Post::Architecture in Msf::Post::Linux::Kernel.
• Replace the kernel_arch implementation with an alias to get_os_architecture.

Impact Analysis:

• Blast radius: medium—affects all code calling kernel_arch via Msf::Post::Linux::Kernel, plus existing unit tests for this mixin; downstream consumer behavior depends on whether callers assume kernel_arch works without an attached session (Unknown beyond identified spec usage).
• Data and contract effects: kernel_arch now inherits get_os_architecture’s contract (uses session/sysinfo; may return nil per its documentation), which differs from the prior always-string behavior.
• Rollback and test focus: run the existing spec/lib/msf/core/post/linux/kernel_spec.rb and validate common Linux modules that call kernel_arch still behave correctly across meterpreter + shell sessions.