Add MySQL changelog to support and fix MySQL CI

[Joshun]

• MySQL support mentioned in CHANGES.txt
• Docs updated to mention MySQL is now supported but mention other databases won't work because custom triggers needed (in case someone expects it to work on other databases that Django supports
• Workaround Github actions CI failing after MariaDB updated their docker image (healthcheck which determines whether MariaDB is running has changed so will need to investigate, but there may be some merit in pinning to a specific version anyway)

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.02 :tada:

Comparison is base (322c264) 92.95% compared to head (d0070f5) 92.97%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #97 +/- ## ========================================== + Coverage 92.95% 92.97% +0.02% ========================================== Files 59 59 Lines 3817 3829 +12 Branches 245 250 +5 ========================================== + Hits 3548 3560 +12 Misses 224 224 Partials 45 45 ``` | [Impacted Files](https://app.codecov.io/gh/adamcharnock/django-hordak/pull/97?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Adam+Charnock) | Coverage Δ | | |---|---|---| | [hordak/models/core.py](https://app.codecov.io/gh/adamcharnock/django-hordak/pull/97?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Adam+Charnock#diff-aG9yZGFrL21vZGVscy9jb3JlLnB5) | `98.31% <100.00%> (+0.08%)` | :arrow_up: | | [hordak/tests/models/test\_core.py](https://app.codecov.io/gh/adamcharnock/django-hordak/pull/97?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Adam+Charnock#diff-aG9yZGFrL3Rlc3RzL21vZGVscy90ZXN0X2NvcmUucHk=) | `100.00% <100.00%> (ø)` | |

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[PetrDlouhy]

@Joshun Thank you very much. I merged this and released django-hordak 1.14.0