ci: update stale.yml GitHub action

[hugo-sid]

What problem does this PR solve?

Mark issues and PRs inactive for 21 days (instead of current setup - 7 days) as stale.

Is this PR adding a new feature?

No.

Is this PR related to any issue or discussion?

No.

PR Checklist

• I have verified that the code works as described/as intended.
• This change adds a social icon which has a permissive license to use it.
• This change does not include any external library/resources.
• This change does not include any unrelated scripts (e.g. bash and python scripts).
• I have enabled maintainer edits for this PR.

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[sonarcloud]

Kudos, SonarCloud Quality Gate passed!   

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