We had removed the dedicated `MysqlDateTime`, `MysqlJson`, and
`OID::Json` classes in the past (f1a0fa9e, #29666), so legacy YAML
loading has no longer always perfectly compatiblity.

Fortunately, v2 (Rails 5.1 style) YAML doesn't contain type information
in almost all cases (unless serializing object using custom select), so
usually removing dedicated type affects to legacy YAML older than v1
(Rails 5.0 style) YAML only.

This restores legacy YAML compatibility for MySQL with v1 format by
adding the class alias in YAML for `MysqlString` which is most recently
reported about compatibility concern. It also affects to legacy Rails
4.2 style YAML, but 4.2 style YAML had already broken by removing
`MysqlDateTime` over 4 years ago.