Cannot start WorkflowHost with MySql on NET6, DbContext injection missing

[lfbertucci]

Hello Daniel,

After the changes on WorkflowCore to run with NET6, MySQL provider stopped working, throwing an error when trying to start de WorkflowCore Host.

The issue only occurs when using MySQL provider with NET6 projects.

After investigating, I found a condition in "MysqlContext" class that validate only NetStandard.

So I'm making a PR and adding the NET6 verification.

Thanks

PS: sorry for open a new PR, I commited some inaproprieted changes at my master and can't revert than, so I deleted my repository and the old PR was closed automatically (old PR #1034 )