use gh cli to rename default branch from master to main in all your repos

rename.sh

#!/usr/bin/env bash
set -euo pipefail

user=ben0x539
repos=(
  $(
    gh repo list --json name,defaultBranchRef --source \
      | jq --raw-output '.[] | select(.defaultBranchRef.name == "master") | .name'
  )
)

for repo in "${repos[@]}"; do
  gh api --silent -X POST -H 'Accept: application/vnd.github.v3.raw+json' "repos/$user/$repo/branches/master/rename" -F new_name=main
done