Simplify tab completion of extension versions, redux.

Simplify tab completion of extension versions, redux.

After commit 662dbe2c8, psql tab completion didn't conveniently
support the case of "ALTER EXTENSION foo UPDATE".  It'd always
add "TO", which is fine if you want to specify a target version
but not if you don't ... and surely the latter is the much more
common case.

To fix, remove "TO" from the initially offered completion; you now
need to press TAB one additional time to get that.  We won't try to
duplicate the old behavior of attempting initial completion on the
target version along with TO.  It's too squirrelly to get the quoting
right, and this is such an infrequent usage that it doesn't seem worth
expending a lot of effort and special code on.

Noted by Noah Misch.  Back-patch to v15.

Discussion: https://postgr.es/m/20220703083217.GB2476530@rfd.leadboat.com