pgsql: Add back vacuum_cleanup_index_scale_factor parameter.

Add back vacuum_cleanup_index_scale_factor parameter.

Commit 9f3665fb removed the vacuum_cleanup_index_scale_factor storage
parameter. However, that creates dump/reload hazards when moving across
major versions.

Add back the vacuum_cleanup_index_scale_factor parameter (though not the
GUC of the same name) purely to avoid problems when using tools like
pg_upgrade. The parameter remains disabled and undocumented.

No backpatch to Postgres 13, since vacuum_cleanup_index_scale_factor was
only disabled by REL_13_STABLE's version of master branch commit
9f3665fb in the first place -- the parameter already looks like this on
REL_13_STABLE.

Discussion: https://postgr.es/m/YEm/a3Ko3nKnBuVq(at)paquier(dot)xyz

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/effdd3f3b633e88feaa675377075f02ecc99aee4

Modified Files
--------------
src/backend/access/common/reloptions.c | 9 +++++++++
src/backend/access/nbtree/nbtutils.c | 2 ++
src/include/access/nbtree.h | 1 +
3 files changed, 12 insertions(+)