Takes authors of last commit (push or pull request) and adds them to CODEOWNERS file per each file. Will keep the last *n of committers on the list per file.

Write a .yml file such as this one in your .github/workflows folder. Refer to the documentation on workflow YAML syntax here.

name: 'update-codeowners'
on:
  pull_request:
    branches:
      - main
  push:
    branches:
      - main
      - 'releases/*'

jobs:
  update:
    runs-on: ubuntu-latest
    # Read the notes about preventing merges overwriting owners created by PR
    if:  ${{ !startsWith( github.event.head_commit.message, 'Merge pull request' ) }}
    steps:
      - uses: actions/checkout@master
        with:
          fetch-depth: 0
      - name: create CODEOWNERS
        id: codeowners
        uses: dmikov/github-action-codeowners-last@main
        with:
          number_of_code_owners: 2
      - name: commit CODEOWNERS
        if: ${{ steps.codeowners.outputs.file }}
        uses: EndBug/add-and-commit@v5
        with:
          message: 'commit CODEOWNERS'
          add: ${{ steps.codeowners.outputs.file }}
        env:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

Action was made intentionally to have a single responsibility, it will add authors when you tell it to. So prevention of double run on PR and merge to main branch is your responsibility. You can of course disallow pushes to main branch without PR (as you should), then you can exclude the main branch from push trigger. As an opposite you can only update codeowners on push to main, less observable, but less of a chance for conflicts between multiple branches. If you need both however, the condition on the job if: github.event.pull_request.merged != true should prevent double runs. Might not be so.

The file has to be committed back, so for the next step its location is helpful.