Add GH action to close stale PRs

[kachawla]

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Description

To declutter the PR queue and ensure that active contributions receive attention, enabling https://github.com/actions/stale github action to automatically close stale PRs after 90 days of inactivity, with a notification period of 7 days.

This will provide contributors with a reasonable window to address any outstanding feedback or issues before the PR is automatically closed. We can adjust this timeframe in the future based on the feedback and learnings. We can also consider PRs labeled with certain labels to be excluded from this workflow if we see the need for this in the future: https://github.com/actions/stale?tab=readme-ov-file#exempt-pr-labels.

Issue reference

Fixes: radius-project/radius#7151