add MariaDB to requirements

[robertsilen]

add MariaDB to requirements, as discussed in #5237

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[bobapple]

@cla-bot check

[nilmerg]

Thank you :D Though, I'm not sure why you picked such an old state. Please adjust this based on main.

[robertsilen]

The 2.8 installation doc implicitly mentions MySQL in the requirements, so I was trying to add MariaDB there, but looks like this PR is going into main now.

For main's current installation doc I suggest this PR: #5247

Sorry, not sure how to best merge or replace these PRs. This PR can be closed if you are not accepting it into 2.8?

[nilmerg]

2.8 is not supported anymore and as such, changes won't be released nor be included in our official documentation.