Add kernel_rex_version for kernel parsing

[EclipseAditya]

Adds kernel_rex_version to Msf::Post::Linux::Kernel that extracts the upstream kernel version from uname -r and returns a Rex::Version. This replaces the split('-').first pattern duplicated across 15 modules that crashed with ArgumentError on distro-specific suffixes (Amazon Linux, Fedora, RHEL, SUSE, etc.).

• Modules now return CheckCode::Unknown instead of CheckCode::Safe when version can't be parsed
• Removes rescue ArgumentError band-aids added in guard Rex::Version.new against crashes on local modules #19813
• Fixes vmwgfx_fd_priv_esc boundary (> to >=) that excluded kernel 4.14.0
• Fixes cve_2021_3490 Fedora dead code (>= to > for 5.11.20)

Fixes #19855

Verification

• bundle exec rspec spec/lib/msf/core/post/linux/kernel_spec.rb
• Verify 34 examples, 0 failures
• bundle exec ruby msftidy.rb vmwgfx_fd_priv_esc.rb docker_cgroup_escape.rb modules/exploits/example_linux_priv_esc.rb
• Verify no new offenses
• grep -rn "Rex::Version.new.*kernel_release" modules/exploits/
• Verify zero matches (no module passes raw kernel_release to Rex::Version)
• Start msfconsole
• irb
• Rex::Version.new("4.14.355-275.572.amzn2.x86_64")
• Verify it raises ArgumentError (the original bug)
• Rex::Version.new("4.14.355-275.572.amzn2.x86_64".split('-').first)
• Verify returns Rex::Version "4.14.355" (what kernel_rex_version does)
• use exploit/linux/local/vmwgfx_fd_priv_esc
• Verify module loads without errors