Add new lock GH action

[juliasilge]

Addresses #4143

This PR uses this very broadly used GH action to lock closed issues that have had no activity for 14 days.

It does not change the behavior around locking merged/deleted PRs, which is discussed more here.

QA Notes

This GH action will help us keep folks from commenting on old, closed issues.

Caution

We have a lot (I mean, tons and tons) of old, closed issues. This action will make a comment on every single one of these and generate a notification. It will process a bunch each day until it makes it through the whole backlog of closed issues. I've gone through this on a few other large repos and if you have notifications turned on, it is extremely annoying for the first several days while it works through all of them.

Option 1

We leave this as is and just deal with the onslaught of notifications, with the goal of getting to a better stage eventually. The disadvantage here is pretty clear. 😩

Option 2

We take out the comment and only lock, which I believe does not notify, possibly adding the comment in later, like next month sometime, after the backlog is worked through. The disadvantage is that people can feel quite put off when they find an old, locked issue that they feel is relevant to their problem; the comment can help them know what to do.

[juliasilge]

I've heard from a couple of team members who prefer that we do not generate notifications here, so I updated that in c9e5055. I will add a note to the wiki once this is merged in.