Re-introduce pgstat_drop_entry(), keeping ABI compatibility

Re-introduce pgstat_drop_entry(), keeping ABI compatibility

This routine acts as a wrapper of a new pgstat_drop_entry_ext(), used in
the core code with a missing_ok argument.

This includes an update of .abi-compliance-history, removing the latest
entry that has documented the change of pgstat_drop_entry().  This
change is applied across v15~v18.  HEAD keeps pgstat_drop_entry() as
single entry point, with the new missing_ok.

Per discussion with Álvaro Herrera and Lukas Fittl.  This is a follow-up
of 850b9218c8e4.

Discussion: https://postgr.es/m/ajZz_sVJVX7pmPHo@alvherre.pgsql
Backpatch-through: 15-18