[DPE-4977] Run integration tests against Juju 3.6

[shayancanonical]

Issue

We need to soon support this charm on juju 3.6. In order to prepare, we are going to run the integration tests against juju 3.6 on a nightly schedule

Solution

First run juju 3.6 tests on PR to ensure that code is geared towards successful runs
Then use dpw to run juju 3.6 tests only on a nightly schedule

[shayancanonical]

The run for 7107c2a shows that all tests (except test_backups.py on GCP) pass on either arm64 or amd64. As a result, we should run juju 3.6 tests on a nightly schedule and attempt to iteratively debug the issues preventing from fully green pipelines.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.60%. Comparing base (ea798cb) to head (8ad2403).
Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #689   +/-   ##
=======================================
  Coverage   75.60%   75.60%           
=======================================
  Files          12       12           
  Lines        3116     3116           
  Branches      475      475           
=======================================
  Hits         2356     2356           
  Misses        617      617           
  Partials      143      143           

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