Hi @lrong
On your first point,
SonarQube 6.7 is compatible with SonarGraph Integration 3.1.0 as mentionned in the Plugin Matrix I sent you.
On your second point, about MySQL drop, you will find here all the important information and link to the tool we can provide to help you migrate from MySQL to a supported DB provider, before upgrading to 7.9 LTS version of SonarQube. Fyi, MySQL was supported until 7.8 version of SonarQube.
Here below additional information on how to proceed :
HTH,
Carine