Fix a few places that were using appendStringInfo() when they should have
been using appendStringInfoString().  Also some cases of
appendPQExpBuffer() that would have been better suited to use
appendPQExpBufferChar(), and finally, some places that used
appendPQExpBuffer() when appendPQExpBufferStr() would have suited better.

There are no bugs are being fixed here.  The aim is just to make the code
use the most optimal function for the job.

All the code being changed here is new to PG14.  It makes sense to fix
these before we branch for PG15.  There are a few other places that we
could fix, but those cases are older code so fixing those seems less
worthwhile as it may cause unnecessary back-patching pain in the future.

Author: Hou Zhijie
Discussion: https://postgr.es/m/OS0PR01MB5716732158B1C4142C6FE375943D9@OS0PR01MB5716.jpnprd01.prod.outlook.com