pgsql: Report next_multi_offset as bigint in pg_control_checkpoint().

Report next_multi_offset as bigint in pg_control_checkpoint().

Commit bd8d9c9bdfa widened MultiXactOffset to 64 bits, but
pg_control_checkpoint() still handle checkPointCopy.nextMultiOffset as
xid type and declared next_multi_offset column as xid. Since xid is 32
bits wide, an offset above 2^32 was reported truncated, while
pg_controldata printed the full value of the same field.

This commit reports the column as bigint instead. That matches
pg_get_multixact_stats(), which already reports num_members and
members_size, both derived from these same offsets, as int8.

Backpatch to v19, where MultiXactOffset was widened.

Bump catalog version.

Reviewed-by: Heikki Linnakangas <hlinnaka(at)iki(dot)fi>
Reviewed-by: Chao Li <li(dot)evan(dot)chao(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAD21AoCvzerscfU8o4ARQ793yAGHpQ72r2x5apeC_W2-k=SLCQ@mail.gmail.com
Backpatch-through: 19

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/45817676632b46fc9d2b97c8d432348e0ee4f1f4

Modified Files
--------------
doc/src/sgml/func/func-info.sgml | 2 +-
src/backend/utils/misc/pg_controldata.c | 2 +-
src/include/catalog/catversion.h | 2 +-
src/include/catalog/pg_proc.dat | 2 +-
4 files changed, 4 insertions(+), 4 deletions(-)