Fix Postgres time shifting problems

[villebro]

The time grain functions on postgres have an unnecessary AT TIME ZONE 'UTC' cast which causes the following problems:

1. Causes timestamps with a non-UTC timezone to be shifted by the UTC offset.
2. Makes time grained vs non time grained temporal handling inconsistent (no UTC casting when time grain isn't applied).

Fixes #6763 .

[codecov-io]

Codecov Report

Merging #6786 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #6786   +/-   ##
=======================================
  Coverage   56.07%   56.07%           
=======================================
  Files         526      526           
  Lines       23257    23257           
  Branches     2782     2782           
=======================================
  Hits        13042    13042           
  Misses       9806     9806           
  Partials      409      409

Impacted Files	Coverage Δ
superset/db_engine_specs.py	54.11% <ø> (ø)	⬆️

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[villebro]

Ready for review.

[villebro]

Any chance to get this merged? I believe this is one of the last known bugs affecting officially supported sqla dbs.