ci: replace hosted semver with an action

[madhead]

I've noticed, that you are using a self-hosted and publicly accessible semver bash script (static.requarks.io/semver, probably a slightly outdated fork of fsaintjacques/semver-tool) to parse semantic versions.

I propose to use my reusable GitHub Action to accomplish the same task. Using the action has, from my point of view, some benefits:

✔️ No need to host a script anymore
✔️ One less dependency to worry about: the action could use @latest tag, while a hosted script, I guess, requires some sort of a redeployment
✔️ The build would become independent of the site. It includes both possible network issues (like DNS) and site name (who knows if you want a rebranding one day?)
✔️ Using an action just looks like a more straightforward way of doing that.

If you would like to ensure it works the same way as before, I've tested it in my fork:

• A successful run is here
• The workflow I used for the check is here

Thank you!

[madhead]

Hey, @NGPixel, do you think it's a good addition?