[3.15] gh-158399: Remove unused GitHub files from backport branch

[hugovk]

• Issue: Remove unused GitHub files from backport branches #158399

[StanFromIreland]

We can also drop .github/SECURITY.md and arguably .github/CONTRIBUTING.rst? That's one two less things we'd have to maintain, and they're not displayed anywhere.

[hugovk]

I mentioned them in the issue:

Only the main copies of these show up in special places in the GitHub UI, but they can stay as documentation in the other branches

But I don't mind too much either way. What do you prefer?

[StanFromIreland]

Oops, sorry, I didn’t read your issue. I think it’d be easier to remove them. As long as they remain on those branches, we’ll feel pressured to backport changes to them even though there’s no practical benefit, since those files aren’t actually used there.