Documentation cleanup about EOL Kea versions

ARM has sections about years-old Kea versions.

Like this note about what changed in Kea 1.2. It may not be of much interest to anyone.

My proposition is to remove documentation about EOL Kea versions. At the moment of this writing, these versions would be 1.7 and below.

Or, where the information about the actual version of Kea since the EOL versions is still helpful and not mentioned anywhere else, keep it, but remove the part that says "since 1.2" or "as of 1.2" and so on.

For valuable documentation about old Kea versions ("before 1.2", "prior to 1.2"), it would be better suited for KB articles. Because their content doesn't change as much.