use [update-npins](https://github.com/getchoo/update-npins) to automate dependency updates.
alternative renovatebot seemed to mostly support flakes so far, while it mostly seemed better at showing what packages got updated to which version. i figured that seemed not worth fixing for now, so settled for update-npins.
not sure how to test before merge, but i added the deploy token (with write rights) and exposed it as the referenced secret.
closes#65.
Reviewed-on: Fediversity/Fediversity#343
Co-authored-by: Kiara Grouwstra <kiara@procolix.eu>
Co-committed-by: Kiara Grouwstra <kiara@procolix.eu>
