Migrate to the Locker GitHub action for locking closed/stale issues/PRs

[jeffhandley]

This migrates away from the FabricBot implementation of a scheduled search over closed issues and adopts the Locker GitHub action authored by the VS Code team.

The updated fabricbot.json was generated via dotnet/fabricbot-config#83. The workflow file was implemented and tested at https://github.com/dotnet/fabricbot-config/blob/main/.github/workflows/locker.yml.

Since this workflow uses the actions/checkout action, we need to ensure the following GitHub Actions settings are enabled:

1. Allow enterprise, and select non-enterprise, actions and reusable workflows
2. Allow actions created by GitHub

Microsoft Reviewers: Open in CodeFlow

[jeffhandley]

FYI @wtgodbe

[ReubenBond]

@jeffhandley do we need to do anything other than merge this?

[jeffhandley]

Your settings likely already match what's needed since there's an existing workflow that uses the actions/checkout step. Nothing else should be needed for this; it can be merged.

There's only 1 other FabricBot feature in use that needs to be addressed (using 'noActivitySince' in an event filter) before we can apply the full transition away from it to the Policy Service bot.