Snap for v21

[RafaelGSS]

Copying and pasting the checklist from the README:

• [ ] 1. Request a new Track for the "node" Snap in the Snapcraft forum in the "Store requests" section. The track should be the major release line number (e.g. 14). The "node" Snap has fast-track approval and is usually authorized within 24 hours by the administrators. This step needs to be performed in order to upload to a new track. An example of this for 14 can be seen here: https://forum.snapcraft.io/t/track-request-for-node-14-fast-track-please/16842/3
• [ ] 2. Create a new branch in this repository, named nodeXX where XX is the release line number.
• [ ] 3. Edit snapcraft.yaml.sh if required for system configuration required to build the new version. In most cases this is not necessary and the master version can be copied. Where the compiler minimums change, the equivilant changes may need to be made in the script.
• [ ] 4. Edit .github/workflows/cron.yml to add the new release line to the matrix.
• [ ] 5. Start a build (manually, or wait for the GitHub Action to trigger by cron), which will update the snapcraft.yaml file for that branch correctly and push the new branch to https://code.launchpad.net/node-snap where it can be further configured.
• [ ] 6. Navigate to https://code.launchpad.net/node-snap and into the new branch and click on "Create snap package".
• [ ] 7. Manually request new builds for the Snap from the Snap configuration page in Launchpad ("Request builds").

[rvagg]

Someone else want to try running some or all of this process? Might be good to iron out whether credentials and permissions are shared in the appropriate way with Build.