DPE-3392 support rollback for unsupported datadir

[paulomach]

Issue

1. deploy charm with mysql v0
2. refresh charm mysql v1
3. mysql v1 starts and make datadir incompat with prior versions
4. mysql v1 then fail to rejoin the cluster, rendering the unit in blocked state
5. attempt to rollback to previous v0
6. on rollback, mysql v0 fails to start, rendering the unit unusable

Solution

• on rollback (step 5), detect specific unsupported downgrade error
• reset datadir, reinstall mysql and re-add as if was a new instance
• proceed with rollback

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 56.66667% with 26 lines in your changes are missing coverage. Please review.

Project coverage is 66.34%. Comparing base (8bf9fb3) to head (ffa2753). Report is 1 commits behind head on main.

:exclamation: Current head ffa2753 differs from pull request most recent head 4a5e436. Consider uploading reports for the commit 4a5e436 to get more accurate results

Files	Patch %	Lines
src/mysql_vm_helpers.py	41.66%	14 Missing :warning:
src/upgrade.py	64.28%	7 Missing and 3 partials :warning:
src/charm.py	60.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #401 +/- ## ========================================== + Coverage 65.90% 66.34% +0.44% ========================================== Files 17 17 Lines 3103 3153 +50 Branches 411 418 +7 ========================================== + Hits 2045 2092 +47 + Misses 929 927 -2 - Partials 129 134 +5 ```

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