fix: Missing replacement of connection with session for ORM query

[fregataa]

fix: lablup/backend.ai#393
follow-up-for: #541

SQLAlchemy connection: https://docs.sqlalchemy.org/en/14/core/connections.html#sqlalchemy.engine.Connection
SQLAlchemy session: https://docs.sqlalchemy.org/en/14/orm/session_api.html#sqlalchemy.orm.Session

Connection is a functionality for DB-API, while Session can manage operations for ORM-mapped objects.
Session should be used when we want to use ORM functionalities.

[codecov]

Codecov Report

Merging #553 (5d2dc0b) into main (7f7a49f) will decrease coverage by 1.83%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #553      +/-   ##
==========================================
- Coverage   49.66%   47.83%   -1.84%     
==========================================
  Files          55       55              
  Lines        8946     9701     +755     
==========================================
+ Hits         4443     4640     +197     
- Misses       4503     5061     +558     

Impacted Files	Coverage Δ
src/ai/backend/manager/models/image.py	49.38% <0.00%> (-0.13%)	⬇️
src/ai/backend/manager/plugin/error_monitor.py	45.76% <0.00%> (-20.34%)	⬇️
src/ai/backend/manager/scheduler/predicates.py	51.85% <0.00%> (-0.68%)	⬇️
src/ai/backend/manager/registry.py	17.42% <0.00%> (-0.29%)	⬇️
src/ai/backend/manager/models/base.py	53.80% <0.00%> (-0.24%)	⬇️
src/ai/backend/manager/server.py	63.85% <0.00%> (+0.16%)	⬆️

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