Create a Github CI action that automatically updates example repos on stable release

[sureshjoshi]

Currently, we have tracking tickets like this #20970

And I've written a script locally which performs the update, and then a PR:
https://gist.github.com/sureshjoshi/2260f08a1d81e7209f7b5d0901c955ad

But that's all pretty overkill for something that we should be able to automate via CI (similar to how we auto-generate docs).

I've never used it, but my guess is we'd use an event like repository_dispatch or some other intra-org, inter-repo event hooks to perform the pants_version update, as well as maybe updating the version of docs that are mentioned (e.g. in the Github actions - https://github.com/pantsbuild/example-visibility/pull/7/files)