apply Grouping on left side of JSONB subscript in compiler

apply Grouping on left side of JSONB subscript in compiler

Fixed regression in PostgreSQL dialect where JSONB subscription syntax
would generate incorrect SQL for :func:`.cast` expressions returning JSONB,
causing syntax errors. The dialect now properly wraps cast expressions in
parentheses when using the ``[]`` subscription syntax, generating
``(CAST(...))[index]`` instead of ``CAST(...)[index]`` to comply with
PostgreSQL syntax requirements. This extends the fix from :ticket:`12778`
which addressed the same issue for function calls.

This reverts how we did the fix for #12778 in Function.self_group()
and instead moves to a direct Grouping() applied in the PG compiler
based on isinstance of the left side.

in retrospect, when we first did #10927, we **definitely** made
the completely wrong choice in how to do this, the original idea
to detect when we were in an UPDATE and use [] only then was
by **far** what we should have done, given the fact that PG indexes
are based on exact syntax matches.  but since we've made everyone
switch to [] format for their indexes now we can't keep going
back and forth.   even though PG would like [] to be the defacto
syntax it simply is not.    We should potentially pursue a dialect/
create_engine option to switch the use of [] back to -> for
all cases except UPDATE.

Fixes: #13067
Change-Id: I2e0d0f45ebb820d2a8f214550f1d1a500bae223b