There are instructions for this step in the Guide at "Removing Outdated Entries for Multiversion Tests". Immediately after branching for a major release (e.g 8.0, 9.0, etc) or a minor release to on-prem, do the following in backports_required_for_multiversion_tests.yml on the master branch of 10gen/mongo:

• If major: under “last-continuous” and “last-lts”, delete everything between “all:” and “suites: null”
  
• If minor: under “last-continuous” only, delete everything between “all:” and “suites: null”
  
  NOTE: this needs to be done after the FCV upgrade to the next version happens on master