Add GitHub template and workflows needed on the default branch

[nickrobinson251]

We want to set the default branch on this repo to be the version that we're currently using, which is v1.9.2+RAI, but the PR template and the workflows that run on a schedule must be in the default branch... so this PR adds those GitHub things to v1.9.2+RAI.
this PR

• copies the files from master+RAI (current default)
• removes the codeowerns file that doesn't apply to this fork (we could always add our own one back later)
• adds a workflow for closing stale PRs [this change can move to it's own PR if you prefer?]

in future, when we change default branch, it should be a simple as copying across the .github/ directory, e.g.

git checkout new-default-branch                # switch to new branch
git checkout old-default-branch -- .github/    # copy across the files from the old branch
git commit -m "add github setup needed when default branch"