Add a tab for CS9
    
    We're still fighting the community perception that CentOS Stream doesn't
    have major versions.  This change adds a 9 tab on the CentOS Stream
    download page.  It's still too early for download links, so instead it
    says "coming soon".  For the same reason, the 8 tab is still the initial
    active tab.  This also moves the CL8 to CS8 conversion instructions to
    only be shown on the 8 tab, as they won't be relevant for 9.