Add rdbms matrix for wider compatibility testing

[WyriHaximus]

This change set introduces a rdbms matrix with the purpose of increasing insurance this package works with up to the latest MySQL/MariaDB versions.

Note that due to MySQL's versioning there are no v6 and v7.

Refs: #194

[SimonFrings]

Interesting approach, but I'm not sure if we should add this to the same MySQL job here. You suggested to test this with mariadb:10 across all supported PHP versions, but I don't think this is necessary here. Looking further, if we want to support additional databases in different versions, this would result in a huge (and completely confusing) test matrix in GitHub actions.

I think we should test mysql across all supported PHP versions, but for mariadb we could also test against the latest PHP version(s) in a different job to avoid overloading this.

What do you think?

[WyriHaximus]

Interesting approach, but I'm not sure if we should add this to the same MySQL job here. You suggested to test this with mariadb:10 across all supported PHP versions, but I don't think this is necessary here. Looking further, if we want to support additional databases in different versions, this would result in a huge (and completely confusing) test matrix in GitHub actions.

We could introduce MariaDB in a follow up PR.

I think we should test mysql across all supported PHP versions, but for mariadb we could also test against the latest PHP version(s) in a different job to avoid overloading this.

Givin that MariaDB is a fork by the creator on MySQL, we could flip a could and prefer one of them?

What do you think?

To be brutally honest: Test all permutations; but that is a personal preference that I enforce in my own packages and projects, so I'll make the adjustment for this PR.

[WyriHaximus]

@SimonFrings Pushed the changes