Add .abi-compliance-history to back-branches.

Add .abi-compliance-history to back-branches.

This file was previously added to v18 by commits a72f7d97be and
93fb76ca4e.  Unlike the v18 version of the file, the back-branch
versions set the original baseline point to the most recent ABI
break documented in the git commit history.  While we'd ordinarily
set it to something just before the .0 release, we're unlikely to
act upon ABI breaks in released minor versions, so it doesn't seem
worth the trouble to construct a comprehensive history.

Discussion: https://postgr.es/m/aPfDOD6F4FaJJd7M%40nathan
Backpatch-through: 13-17