Migrate away from mysql 5.7

[tobil4sk]

https://github.com/HaxeFoundation/haxelib/actions/runs/7431993301/job/20223297439#step:5:5345

W: GPG error: http://repo.mysql.com/apt/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY B7B3B788A8D3785C

mysql 5.7 is no longer supported so I don't think it will be fixed. We might have to upgrade to mysql 8.

A new replacement key (A8D3785C) will sign upcoming MySQL 8.0.36 and higher packages

https://forums.mysql.com/read.php?11,712128,712680#msg-712680

[tobil4sk]

@andyli any idea how to deal with this? It is preventing progress with #619

[tobil4sk]

The CI failure is avoided for now thanks to #626, but we will still have to migrate away from mysql 5.7 at some point in the near future.