ci: use stale action to apply needs-attention label

[skl]

Inspired by the grafana/alloy workflow:

• https://github.com/grafana/alloy/blob/main/.github/workflows/needs-attention.yml

The config in this PR:

• marks issues and PRs with the label needs-attention (unless already labelled with keepalive)
• issues and PRs are not closed automatically
• runs daily at midnight (see https://crontab.guru/#0_0_*_*_*)

[skl]

@povilasv see 9fe1b34 which updates the stale action to auto-close issues and PRs

[povilasv]

Thank you!