Node 16 is deprecated, update to node 20

[e12e]

Running action with debug logging, we get a warning:

Node.js 16 actions are deprecated. Please update the following actions to use Node.js 20: aki77/simplecov-report-action@v1. For more information see: https://github.blog/changelog/2023-09-22-github-actions-transitioning-from-node-16-to-node-20/.

In theory it should be enough to change:

https://github.com/aki77/simplecov-report-action/blob/master/action.yml#L20

from:

runs:
  using: "node16"

to:

runs:
  using: "node20"