Add current_timestamp to Redshift as a bare function

[RossOkuno]

Brief summary of the change made

This PR is to allow current_timestamp without parenthesis in Redshift.
Fixes #3573

Are there any other side effects of this change that we should be aware of?

None

Pull Request checklist

• Please confirm you have completed any of the necessary steps below.

• Included test cases to demonstrate any code changes, which may be one or more of the following:

  • .yml rule test cases in test/fixtures/rules/std_rule_cases.
  • .sql/.yml parser test cases in test/fixtures/dialects (note YML files can be auto generated with tox -e generate-fixture-yml).
  • Full autofix test cases in test/fixtures/linter/autofix.
  • Other.

• Added appropriate documentation for the change.

• Created GitHub issues for any relevant followup/future enhancements if appropriate.

[codecov]

Codecov Report

Merging #3741 (b94f0d6) into main (bc14733) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main     #3741   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          176       176           
  Lines        13466     13466           
=========================================
  Hits         13466     13466           

Impacted Files	Coverage Δ
src/sqlfluff/dialects/dialect_redshift.py	100.00% <100.00%> (ø)

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[tunetheweb]

LGTM!