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.