Move the newly defined enum value REORDER_BUFFER_CHANGE_INTERNAL_SPEC_ABORT
at the end to avoid ABI break in the back branches. We need to back-patch
this till v11 because before that it is already at the end.
Reported-by: Tomas Vondra
Backpatch-through: 11
Discussion: https://postgr.es/m/CAExHW5sPKF-Oovx_qZe4p5oM6Dvof7_P+XgsNAViug15Fm99jA@mail.gmail.com