The Mysql repository does not support jessie anymore.
Solution: change jessie to buster in your /etc/apt/sources.list.d/mysql.list, so it looks like this:
deb [trusted=yes][signed-by=/usr/share/keyrings/mysql-apt-config.gpg] http://repo.mysql.com/apt/debian/ buster mysql-apt-config
deb [trusted=yes][signed-by=/usr/share/keyrings/mysql-apt-config.gpg] http://repo.mysql.com/apt/debian/ buster mysql-5.7
deb [trusted=yes][signed-by=/usr/share/keyrings/mysql-apt-config.gpg] http://repo.mysql.com/apt/debian/ buster mysql-tools
deb-src [trusted=yes][signed-by=/usr/share/keyrings/mysql-apt-config.gpg] http://repo.mysql.com/apt/debian/ buster mysql-5.7