Here’s the brute-force approach. It also removes the configuration of the repository.
Note: This does NOT work if the repository has submodules! If you are using submodules, you should use e.g. interactive rebase
Step 1: remove all history (Make sure you have backup, this cannot be reverted)
cat .git/config # note <github-uri>
rm -rf .git
Step 2: reconstruct the Git repo with only the current content
git init
git add .
git commit -m "Initial commit"
Step 3: push to GitHub.
git remote add origin <github-uri>
git push -u --force origin main