Upgrade MySQL to 8.0

[gkcalat]

As MySQL 8 is a default version in GCP and katib, it was proposed to upgrade MySQL version to 8.0.

    cc @gkcalat for upgrading MySQL version in the future release.

Originally posted by @zijianjoy in #382 (comment)

Additional docs:

• MySQL 8.0 release notes
• Instruction on upgrading SQL
• GCP MySQL versions
• GCP supported machine types
• GCP DB settings

Consideration:

• Is it a breaking change for users using KF 1.6.0 and prior?
• Do we need to upgrade tier?
• Do we need to enforce utf8 charset as MySQL8 migrated to utf8mb4?

[gkcalat]

/cc @zijianjoy

[zijianjoy]

FYI, MLMD and Argo have already migrated to MySQL 8. And KFP on AI Platform Pipelines also allows us to configure MySQL 8 connection too. That means that the usability on MySQL 8 should be verified already. However, we need to do the followings:

1. Upgrade KFP base mysql deployment to v8: https://github.com/kubeflow/pipelines/blob/master/manifests/kustomize/third-party/mysql/base/mysql-deployment.yaml#L29
2. Validate the upgrade from v5 to v8 in full Kubeflow mode so we can determine whether it is a breaking change.
3. I think there should still be some degree of breaking changes any way, so we need to update documentation about it.