Use juju 3 in integ tests

[nrobinaubertin]

Overview

Adapt integration tests to run on juju 3.

Checklist

• The charm style guide was applied
• The contributing guide was applied
• The changes are compliant with ISD054 - Managing Charm Complexity
• The documentation is generated using src-docs
• The documentation for charmhub is updated.
• The PR is tagged with appropriate label (urgent, trivial, complex)

[github-actions]

Test coverage for 4daf4ac

Name              Stmts   Miss Branch BrPart  Cover   Missing
-------------------------------------------------------------
src/charm.py        324     32     84     12    89%   137, 163, 173, 181-182, 338->343, 526-528, 533-534, 545-547, 552-553, 565-567, 572-573, 585-587, 612-614, 654->exit, 664->exit, 694-700, 726->exit, 740-741, 751->exit, 765
src/database.py      30      1      8      1    95%   56
-------------------------------------------------------------
TOTAL               354     33     92     13    90%

Static code analysis report

Run started:2023-11-30 20:57:16.134030

Test results:
  No issues identified.

Code scanned:
  Total lines of code: 1905
  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):

[cbartz]

LGTM