ci: Add github stale action

[goatgoose]

Resolved issues:

None

Description of changes:

The github stale app has not been functioning. Other stale app users seem to have similar problems, see probot/stale#384. Github now recommends the stale action rather than the stale app, see probot/probot.github.io#376. This PR removes the stale app config and adds the stale action instead.

Call-outs:

If the stale action is modified in a PR, a stale action is added to the PR with dry-run mode enabled, which outputs the changes to the log without actually changing anything.

Testing:

See the output in the "Close stale PRs" action:

Processed items: 412
├── Processed issues: 383
└── Processed PRs   : 29
New stale PRs: 8
Added PRs labels: 8
Added PRs comments: 8
Fetched items: 412
Fetched items events: 8
Fetched items comments: 8
Operations performed: 38

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[dougch]

thanks for looking at this