Remove ops pg lib and pin dependencies

[arturo-seijas]

The ops-lib-pgsql is not needed anymore, as the legacy db relation has been removed

[github-actions]

Test coverage for 90ffaa0

Name                            Stmts   Miss Branch BrPart  Cover   Missing
---------------------------------------------------------------------------
src/actions/__init__.py             3      0      0      0   100%
src/actions/register_user.py       20      0      2      0   100%
src/actions/reset_instance.py      21      3      2      1    83%   54-58
src/charm.py                       83      2      8      1    97%   139-140
src/charm_state.py                 45      2     10      2    93%   22, 79
src/charm_types.py                  8      0      0      0   100%
src/constants.py                   11      0      0      0   100%
src/database_client.py             53      1     10      3    94%   35, 47->exit, 69->exit
src/database_observer.py           54      4      6      0    93%   70-72, 96
src/exceptions.py                   4      0      0      0   100%
src/observability.py                9      0      0      0   100%
src/pebble.py                      47      0      0      0   100%
src/synapse/__init__.py             3      0      0      0   100%
src/synapse/api.py                 46      2      2      1    94%   126-127
src/synapse/workload.py            87      3     10      1    96%   147, 278-283
src/user.py                        23      0      4      0   100%
---------------------------------------------------------------------------
TOTAL                             517     17     54      9    95%

Static code analysis report

Run started:2023-08-16 14:47:14.431059

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 2386
  Total lines skipped (#nosec): 1
  Total potential issues skipped due to specifically being disabled (e.g., #nosec BXXX): 0

Run metrics:
  Total issues (by severity):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
  Total issues (by confidence):
  	Undefined: 0
  	Low: 0
  	Medium: 0
  	High: 0
Files skipped (0):