fix(db2): change engine name and add legacy alias

[villebro]

SUMMARY

Currently we only support the legacy engine name ibm_db_sa for DB2. This changes the primary name of the DB2 engine to db2 and adds the ibm_db_sa alias as a fallback per the comment here:

https://github.com/ibmdb/python-ibmdbsa/blob/7c7cef341829a316bf46561920aa5a789f498431/ibm_db_sa/setup.py#L42-L57

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

TEST PLAN

ADDITIONAL INFORMATION

• Has associated issue: closes There is no option in TIME GRAIN #14263

Ping @ljclour

[codecov]

Codecov Report

Merging #14295 (573e7f8) into master (5d3191b) will increase coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #14295      +/-   ##
==========================================
+ Coverage   76.93%   77.01%   +0.08%     
==========================================
  Files         954      954              
  Lines       48070    48071       +1     
  Branches     5973     5973              
==========================================
+ Hits        36982    37022      +40     
+ Misses      10886    10847      -39     
  Partials      202      202              

Flag	Coverage Δ
hive	80.44% <100.00%> (+<0.01%)	⬆️
mysql	80.73% <100.00%> (+<0.01%)	⬆️
postgres	80.76% <100.00%> (+0.01%)	⬆️
presto	80.46% <100.00%> (?)
python	81.30% <100.00%> (+0.15%)	⬆️
sqlite	80.37% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
superset/db_engine_specs/db2.py	91.66% <100.00%> (+0.75%)	⬆️
superset/models/core.py	89.13% <0.00%> (+0.27%)	⬆️
superset/connectors/sqla/models.py	90.07% <0.00%> (+1.45%)	⬆️
superset/db_engine_specs/presto.py	89.89% <0.00%> (+5.47%)	⬆️

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