Uninstalling php5 and libapache2-mod-php5 (check exact package names with apt-cache search) should do it.
Make sure you don’t have any old PHP code that breaks in PHP7 : unsafe features get deprecated on PHP upgrades, but there’s always an alternative and better method when that happens. I had that problem going from 7.0 to 7.3, in particular with PCRE matching becoming less error tolerant.
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