Replace deprecated keepalive action with gh api call

[jaimergp]

Closes #313

[ocefpaf]

Thanks for fixing this Jaime!

[wolfv]

Should we do the same for the conda-mirror job?

[jaimergp]

Yep, we have a couple more places with this deprecated action. I think I'll create a centralized action for us to reuse internally so we don't have to maintain copy-pasted code in several places.

[jaimergp]

Actually just forked https://github.com/liskin/gh-workflow-keepalive to https://github.com/conda-forge/keepalive-action.

[jaimergp]

See:

• Remove keep-alive jobs channel-mirrors/mirrormirror#26
• Use conda-forge/keepalive-action #316