fix(druid): Fix regression with ISO 8601 format

[john-bodley]

SUMMARY

This PR fixes a regression I introduced in #17050 as it seems like the P0.25Y ISO8601 isn't supported,

druid> SELECT TIME_FLOOR(TIME_PARSE('2020-10-12'), 'P0.25Y')
Unknown exception (java.lang.IllegalArgumentException): Invalid format: "P0.25Y"

yet per the Druid SQL documentation for TIME_FLOOR

Rounds down a timestamp, returning it as a new timestamp. Period can be any ISO8601 period, like P3M (quarters) or PT12H (half-days).

it explicitly states any ISO8601 format is acceptable. This change uses P3M instead (which is the direction o #17078).

TESTING INSTRUCTIONS

druid> SELECT TIME_FLOOR(TIME_PARSE('2020-10-12'), 'P3M')
2020-10-01T00:00:00.000Z

ADDITIONAL INFORMATION

• Has associated issue:
• Required feature flags:
• Changes UI
• Includes DB Migration (follow approval process in SIP-59)
  • Migration is atomic, supports rollback & is backwards-compatible
  • Confirm DB migration upgrade and downgrade tested
  • Runtime estimates and downtime expectations provided
• Introduces new feature or API
• Removes existing feature or API

[codecov]

Codecov Report

Merging #17079 (46907dc) into master (5e85f48) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #17079      +/-   ##
==========================================
- Coverage   76.88%   76.81%   -0.08%     
==========================================
  Files        1031     1031              
  Lines       55183    55183              
  Branches     7505     7505              
==========================================
- Hits        42430    42388      -42     
- Misses      12501    12543      +42     
  Partials      252      252              

Flag	Coverage Δ
hive	81.47% <ø> (ø)
javascript	70.82% <ø> (ø)
mysql	81.92% <ø> (ø)
postgres	81.93% <ø> (ø)
presto	?
python	82.28% <ø> (-0.15%)	⬇️
sqlite	81.60% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
superset/db_engine_specs/druid.py	86.27% <ø> (ø)
superset/db_engine_specs/presto.py	84.30% <0.00%> (-5.65%)	⬇️
superset/connectors/sqla/models.py	85.83% <0.00%> (-1.42%)	⬇️
superset/models/core.py	89.26% <0.00%> (-0.74%)	⬇️

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[graceguo-supercat]

LGTM