This way we don't have to update the text and all translations every time we
bump the version.
Remove the year from the error text, it's cumbersome to update and I don't find
it very important to have in the message.
Also remove the invitation to file an issue; I don't find it very likely that we
are going to relax the minimum git requirement again.
We've been sometimes using lo and sometimes using my slices package, and we need to pick one
for consistency. Lo is more extensive and better maintained so we're going with that.
My slices package was a superset of go's own slices package so in some places I've just used
the official one (the methods were just wrappers anyway).
I've also moved the remaining methods into the utils package.
