Remove transaction isolation level config in PG

[suddendust]

Description

This PR removes poolableConnectionFactory.setDefaultTransactionIsolation(TRANSACTION_READ_COMMITTED);. TRANSACTION_READ_COMMITTED is already the default - However, setting this setting it explicitly causes DBCP2 to execute SHOW TRANSACTION ISOLATION LEVEL on every connection borrow which is highly sub-optimal. We observed that with this configuration, getPooledConnection is almost ~ 50% of the wall-clock time in executeUpsert call. However, with this removed, it's ~ 0.46%.

With Config:

Without Config (the frame for getPooledConnection is not visible since it's so small):

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.73%. Comparing base (daf5ccb) to head (e3cabb0).

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #306      +/-   ##
============================================
- Coverage     81.73%   81.73%   -0.01%     
  Complexity     1532     1532              
============================================
  Files           241      241              
  Lines          7413     7412       -1     
  Branches        717      717              
============================================
- Hits           6059     6058       -1     
  Misses          908      908              
  Partials        446      446              

Flag	Coverage Δ
integration	81.73% <ø> (-0.01%)	⬇️
unit	55.78% <ø> (-0.01%)	⬇️

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[github-actions]

Test Results

  122 files  ±0    122 suites  ±0   39s ⏱️ ±0s
  829 tests ±0    828 ✅ ±0  1 💤 ±0  0 ❌ ±0 
1 166 runs  ±0  1 165 ✅ ±0  1 💤 ±0  0 ❌ ±0 

Results for commit e3cabb0. ± Comparison against base commit daf5ccb.