Reduce BWC testing to include only latest patch releases (#106143)

This commit alters our backward compatibility testing logic to restrict
testing to the latest patch release for any given minor version. Since
our policy is not to do any kind of breaking changes in patches, most of
this testing is superfluous. This change reduces the number of versions
we need to test by over 70%.

(cherry picked from commit b1e4407)

Author: mark-vieira