I want to update mysql vertion 5.5.59 to 5.7.21
First, never try to uninstall or purge your mysql packages. Your db be drop from your system for ever…

Secondly, you should try to use apt-cache policy mysql* to check which versions are available on your system.

Thirdly, you should do a backup of ALL you DB.

If it is a simple update, you should try sudo apt-get upgrade mysql-client mysql-server mysql-common.

Or more aggressive sudo apt-get dist-upgrade mysql-client mysql-server mysql-common

If not, use the version available for update from apt-cache policy mysql* output.

And, from what I can see, your source.list seems not to contain any repo that contains mysql-server-core-5.7. So use apt-cache policy mysql*.