Add .abi-compliance-history to v18 branch.

This is just a quick commit to verify that the buildfarm ABI
checker responds to this control file.  Once we've tested
that, we will update the file to point at c8af5019b.
There's documentation mop-up yet to do, too.

Discussion: https://postgr.es/m/aPJ03E2itovDBcKX@nathan
Backpatch-through: 18 only

.abi-compliance-history

@@ -0,0 +1,7 @@
+# Reference point for ABI compliance checks
+# Add new entries by adding the output of the following to the top of the file:
+# $ git log --pretty=format:"%H # %cd%n# %s" $ABIBREAKGITHASH -1 --date=iso
+#
+# Initial entry would normally point to a commit slightly before the .0 stamp.
+9bbcec6030a2744d83311370ec92213fbd76e514 # 2025-09-22 14:18:56 +0200
+# Translation updates