ci: add stale issue and PR workflow

[adamyeats]

Copies the stale workflow from grafana/grafana verbatim, as part of a rollout across the standalone data source repos.

What it does:

• Issues inactive for 1 year are labelled stale, then closed after a further 30 days of inactivity.
• PRs inactive for 30 days are labelled stale, then closed after a further 2 weeks.
• Apply the no stalebot label to exempt an issue or PR (issues labelled type/epic are also exempt).

User-facing impact: community issues and PRs in this repo will start receiving stale warnings and may be auto-closed by the bot.

[cla-assistant]

All committers have signed the CLA.

[cla-assistant]

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[Copilot]

Pull request overview

Adds an automated GitHub Actions workflow to mark and close stale issues/PRs, aligning this repository with the stale-bot rollout used across Grafana standalone data source repos.

Changes:

• Introduces a scheduled .github/workflows/stale.yml workflow using actions/stale@v10.
• Configures issue staleness at 1 year + 30 days to close, and PR staleness at 30 days + 14 days to close.
• Exempts items with no stalebot (and type/epic for issues).

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[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}