(maint) Pin EL mocks to the ".0" release

In theory, the initial release of EL in each family should
be forward-compatible through the lifecycle. The reverse is
not guaranteed to be true.

This commit pins the EL mocks against the ".0" release
of each EL family to avoid issues with, for example, a
later version of a library adding (or removing) symbols that make
software compiled against it incompatible with earlier versions
of the library.