nostr: avoid a String per generic tag key in Filter - #1427

Node.js 20 is deprecated. The following actions target Node.js 20 but are being forced to run on Node.js 24: actions/checkout@v4, extractions/setup-crate@4993624604c307fbca528d28a3c8b60fa5ecc859, Swatinem/rust-cache@v2.7.8. For more information see: https://github.blog/changelog/2025-09-19-deprecation-of-node-20-on-github-actions-runners/