Provide guidance to RPM users to uninstall old Brim before installing new Brim

[philrz]

If a Linux user is running an RPM-installed Brim version based on the old build/packaging such as the last GA release v0.24.0, then they install a new release RPM based on electron-builder, they'll see this kind of problem:

If they uninstall the old version first, there's no problem.

Right now Linux users are not auto-updated and instead are given a notice about availability of the new version along with a Get Update button that links to https://www.brimsecurity.com/download/. We discussed this as a group and figured that for now we can put some prominent guidance (big red letters, etc.) on the Download page that informs the user of the need to uninstall. I'll also plan to write a Support article in the wiki that describes this in more detail, and we can link to that from the page as well. We can put the same guidance in the Release Notes in the GitHub Releases page as well.

[philrz]

The Brim wiki article at https:/brimdata/brim/wiki/Linux-RPM-Upgrade-%28v0.25.0-%29 now provides this guidance. We'll link to this doc prominently from https://www.brimsecurity.com/download/ and the Release Notes when assembling the upcoming GA Brim v0.25.0.