Bump mysql version to 8 in tests

### Discussed in https://github.com/woodpecker-ci/woodpecker/discussions/2392

<div type='discussions-op-text'>

<sup>Originally posted by **pat-s** September  8, 2023</sup>
Because of https://github.com/woodpecker-ci/woodpecker/pull/2232.

There will most likely never be a arm64 image for mysql 5.x and this DB type/version might anyhow not be the most recommended one anyhow?

</div>