ci: Use custom action to commit changes in CI instead of `git commit` (#792)

We want to enforce commit signing for all commits in our repositories.
To do that, we need to make sure even commits created by CI workflows
are signed.

It would be possible to sign using GPG keys, but that would require a
lot of maintenance.

Instead, we can commit using the GitHub GraphQL API, which automatically
signs commits.

This PR replaces direct `git commit` / `git push` usage (and third-party
commit actions like `EndBug/add-and-commit`)
with the `apify/actions/signed-commit` action, which uses the GraphQL
API under the hood.

As agreed on Slack, I'm also simplifying the workflows to always work on
top of the default branch, rather than on a ref passed down through the
input, as it should effectively be the same.