[Dev] [DB SIG] Step down as DB SIG maintainer

Hi All, I'm Bo Zhao, so apologized that I had to step down as DB SIG maintainer today. I worked in openEuler DB SIG for 1 year. SIG partners worked together and made so many great contributions in openEuler. I'm so happy that I can work with you guys. That's an unforgettable time in life. I will post a PR for the roles removing. Thank you all about your helps and hard work. openEuler will be better and successful. Best Regards, Bo Zhao