Automate stable_abi.txt updates

[messense]

We can create a cron workflow on GitHub Actions that updates stable_abi.txt via pull request automatically.

Resources:

1. https://github.com/marketplace/actions/create-pull-request

[messense]

It'd be nice if we can also update the stable abi exports count in parse_stable_abi_txt test case automatically to avoid manual update.

[ravenexp]

These checks are not essential and were added in the early development stages to test something.
Now that stable_abi.txt is updated semi-automatically, they can be changed from == to >= or even commented out.