Fix incorrect parsing of timestamps in cast from varchar

[martint]

Trailing characters were incorrectly being treated as a timezone and ignored.

Fixes #14164

Release notes

(x) Release notes are required, with the following suggested text:

# General
* Fix incorrect parsing of invalid values in cast from `varchar` to `timestamp` ({issue}`14164`)

[martint]

There's some additional work needed. Per the SQL spec:

1. If the rules for <literal> or for <unquoted timestamp string> in Subclause 5.3, “<literal>”, can be applied to SV to determine a valid value of the data type TD, then let TV be that value.
2. If the rules for <literal> or for <unquoted timestamp string> in Subclause 5.3, “<literal>”, can be applied to SV to determine a valid value of the data type TIMESTAMP(TSP) WITH TIME ZONE, then let TV be the value of:
   CAST ( CAST ( VE AS TIMESTAMP(TSP) WITH TIME ZONE ) AS TIMESTAMP(TSP) WITHOUT TIME ZONE )

Therefore, if the string is a valid timestamp with time zone value, it needs to be parsed as such first and then converted to timestamp without time zone according to the rules of the cast operation between those types.