doc: os recommendations cleanup

This pull request fixes some miscellaneous issues in the os-recommendations documentation page.

The first commit in this PR clarifies the description for the "B" flag in the footnote. We don't exactly do continuous builds on all the platforms marked with "B", but we have published binary RPMs for them. This pull request adjusts the "B" footnote definition to reflect this.

The second commit in this PR removes the old Ceph versions that are no longer supported and includes a note indicating that users should upgrade to supported versions.

The third commit in this PR tweaks the Fedora minor versions. Fedora users don't expect to see ".0" in release numbers.