Bulk update your Netlify build images to Ubuntu Focal. https://docs.netlify.com/configure-builds/overview/#build-image-selection

netlify-build-image-bulk.sh

#!/bin/bash
# Assumes that you have the Netlify and jq CLI tools installed and that you are logged in.
# Only been tested on macOS and zsh
# Will change the build image for all sites in a Netlify account to the specified image
#
# Usage: sh ./netlify-build-image-bulk.sh

# This assumes that you have logged in with the CLI fairly recently and that you're on macOS
# For other systems: https://github.com/sindresorhus/env-paths#pathsconfig
NETLIFY_AUTH=$(cat ~/Library/Preferences/netlify/config.json|jq -r ".users[].auth.token")

# GET THE SITE IDS
echo "Fetching sites. Please wait."
SITE_IDS=$(netlify sites:list --json|jq -r '.[].id')
echo "Done fetching your sites. Moving on to updating their build images."


for SITE_ID in $SITE_IDS
do
  echo "Updating build image for ${SITE_ID}"

  SITE_NAME=$(curl -s "https://app.netlify.com/access-control/bb-api/api/v1/sites/${SITE_ID}" \
     -X 'PUT' \
     -H 'content-type: application/json' \
     -H "cookie: _nf-auth=${NETLIFY_AUTH}; _nf-auth-hint=user-is-likely-authed;" \
     -d '{"build_image":"focal"}'|jq -r '.name')
  echo "Done updating build image for ${SITE_NAME}\n"
  # Be nice with the Netlify API and don't hit rate limits
  sleep 1
done